Pharmacological modulation of voltage-dependent calcium channels in intact cells

Calcium Homeostasis, Transporters, and Blockers in Health and Diseases of the Cardiovascular System

Calcium is a highly positively charged ionic species. It regulates all cell types' functions and is an important second messenger that controls and triggers several mechanisms, including membrane stabilization, permeability, contraction, secretion, mitosis, intercellular communications, and in the activation of kinases and gene expression. Therefore, controlling calcium transport and its intracellular homeostasis in physiology leads to the healthy functioning of the biological system. However, abnormal extracellular and intracellular calcium homeostasis leads to cardiovascular, skeletal, immune, secretory diseases, and cancer. Therefore, the pharmacological control of calcium influx directly via calcium channels and exchangers and its outflow via calcium pumps and uptake by the ER/SR are crucial in treating calcium transport remodeling in pathology. Here, we mainly focused on selective calcium transporters and blockers in the cardiovascular system.

Endomorphin-1 is more potent than endomorphin-2 in inhibition of synaptic transmission in substantia gelatinosa of adult rat spinal cord

Effects of endomorphin-1 (EM-1) and endomorphin-2 (EM-2) on synaptic trans-mission were investigated on neurons in substantia gelatinosa (SG) of the spinal dorsal horn. Both EM-1 (1 microM) and EM-2 (1 microM) remarkably reduced the frequency but not the amplitude of miniature excitatory postsynaptic currents (mEPSCs) and miniature inhibitory postsynaptic currents (mIPSCs). These effects were antagonized by beta-funaltrexamine (beta-FNA, 10 microM), a selective mu-opioid receptor antagonist. Noticeably, EM-1 showed higher potency in decreasing the frequency of mEPSCs and mIPSCs than that of EM-2. These results indicate that EMs suppress both excitatory and inhibitory synaptic transmission by activating presynaptic mu-opioid receptors in the SG and EM-1, compared with EM-2, might be a more potent endogenous analgesic at the spinal cord level.

Raloxifene acutely suppresses ventricular myocyte contractility through inhibition of the L-type calcium current

1. The selective oestrogen (ER) receptor modulator, raloxifene, is widely used in the treatment of postmenopausal osteoporosis, but may also possess cardioprotective properties. We investigated whether it directly suppresses myocyte contractility through Ca(2+) channel antagonism in a similar way to 17beta-oestradiol. 2. Cell shortening and Ca(2+) transients were measured in single guinea-pig ventricular myocytes field-stimulated (1 Hz, 37 degrees C) in a superfusion chamber. Electrophysiological recordings were performed using single electrode voltage-clamp. 3. Raloxifene decreased cell shortening (EC(50) 2.4 microm) and the Ca(2+) transient amplitude (EC(50) 6.4 microm) in a concentration-dependent manner. At a concentration of 1 microm, raloxifene produced a 33+/-2% (mean+/-s.e.m) and 24+/-2% reduction, respectively (P<0.001, n=14 for both parameters). 4. These inhibitory actions were not observed in myocytes that had been incubated with the specific antagonist, ICI 182,780 (10 microm) (n=11). 5. Raloxifene (1 microm) shortened action potential durations at 50 and 90% repolarisation (P<0.05 and <0.001, respectively; n=27) and decreased peak L-type Ca(2+) current by 45%, from -5.1+/-0.5 pA/pF to -2.8+/-0.3 pA/pF (P<0.001, n=18). 6. Raloxifene did not significantly alter sarcoplasmic reticulum Ca(2+) content, as assessed by integrating the Na(+)/Ca(2+) exchanger currents following rapid caffeine application. 7. The present study provides evidence for direct inhibitory actions of raloxifene on ventricular myocyte contractility, mediated through Ca(2+) channel antagonism.

Endomorphins and related opioid peptides

Opioid peptides and their G-protein-coupled receptors (delta, kappa, mu) are located in the central nervous system and peripheral tissues. The opioid system has been studied to determine the intrinsic mechanism of modulation of pain and to develop uniquely effective pain-control substances with minimal abuse potential and side effects. Two types of endogenous opioid peptides exist, one containing Try-Gly-Gly-Phe as the message domain (enkephalins, endorphins, dynorphins) and the other containing the Tyr-Pro-Phe/Trp sequence (endomorphins-1 and -2). Endomorphin-1 (Tyr-Pro-Trp-Phe-NH2), which has high mu receptor affinity (Ki = 0.36 nM) and remarkable selectivity (4000- and 15,000-fold preference over the delta and kappa receptors, respectively), was isolated from bovine and human brain. In addition, endomorphin-2 (Tyr-Pro-Phe-Phe-NH2), isolated from the same sources, exhibited high mu receptor affinity (Ki = 0.69 nM) and very high selectivity (13,000- and 7500-fold preference relative to delta and kappa receptors, respectively). Both opioids bind to mu-opioid receptors, thereby activating G-proteins, resulting in regulation of gastrointestinal motility, manifestation of antinociception, and effects on the vascular systems and memory. To develop novel analgesics with less addictive properties, evaluation of the structure-activity relationships of the endomorphins led to the design of more potent and stable analgesics. Opioidmimetics and opioid peptides containing the amino acid sequence of the message domain of endomorphins, Tyr-Pro-Phe/Trp, could exhibit unique binding activity and lead to the development of new therapeutic drugs for controlling pain.

Effects of azelastine on contractility, action potentials and L-type Ca(2+) current in guinea pig cardiac preparations

Azelastine is used for symptomatic relief of allergic rhinitis and asthma bronchiale. In vitro studies in smooth muscle cells from guinea pig trachea and ileum demonstrate that the drug blocks L-type Ca(2+) current (I(Ca, L)). However, for safety reasons, it is important to know whether azelastine also affects cardiac I(Ca, L) in therapeutically relevant concentrations. We have therefore studied the effects of azelastine on I(Ca, L) in guinea pig ventricular myocytes using standard whole-cell patch-clamp technique. Force of contraction and action potentials from isolated papillary muscles of the same species were also investigated at physiological temperature (36 degrees C). Azelastine (30 microM) significantly reduced force of contraction, shortened action potential duration, and depressed maximum upstroke velocity. I(Ca, L) was elicited by 200-ms-long clamp steps from -100 to 0 mV (one pulse every 3 s). Azelastine blocked I(Ca, L) reversibly and concentration-dependently with an IC(50) of 20.2+/-1.3 microM and a Hill coefficient of 1.1. At 10 microM, azelastine shifted steady-state inactivation by 5 mV (n=7) to more negative potentials. The time course of I(Ca, L) inactivation could be described by a double exponential function. Azelastine (10 microM) significantly shortened the slow inactivation time constant (tau(s)) from 54.2+/-2.8 ms under control conditions to 38.7+/-2.9 ms (n=16) in the presence of drug. Azelastine also reduced low-voltage-activated Ca(2+) currents with a similar IC(50) value (24 microM, at -35 mV). Since the therapeutic plasma concentrations are in the order of 10-100 nM, we conclude that azelastine does indeed affect also cardiac I(Ca, L), but the concentrations required are at least two orders of magnitude larger than those obtained during drug therapy.

Endomorphin-1 and endomorphin-2: pharmacology of the selective endogenous mu-opioid receptor agonists

The recently discovered endogenous opioid peptides, endomorphins-1 and -2, appear to have properties consistent with neurotransmitter/neuromodulator actions in mammals. This review surveys the information gained so far from studies of different aspects of the endomorphins. Thus, the endomorphins have been found unequally in the brain; they are stored in neurons and axon terminals, with a heterogeneous distribution; they are released from synaptosomes by depolarization; they are enzymatically converted by endopeptidases; and they interact specifically and with high affinity with mu-opioid receptors. The most outstanding effect of the endomorphins is their antinociceptive action. This depends on both central and peripheral neurons. Additionally, the endomorphins cause vasodilatation by stimulating nitric oxide release from the endothelium. Their roles in different central and peripheral functions, however, have not been fully clarified yet. From a therapeutic perspective, therefore, they may be conceived at present as potent antinociceptive and vasodilator agents.

Effects of (+) and (-) enantiomers of calcium channel agonist, Bay K 8644, on mechanical and electrical responses of frog skeletal muscle

The effects of (+) and (-) enantiomers of Bay K 8644, a Ca2+ channel agonist, on the mechanical and electrical properties of frog skeletal muscle fibers were investigated. In the concentration range of 10-6 to 10-5 M, both (+) and (-) enantiomers of Bay K 8644 significantly increased the maximum amplitudes of twitch responses. Both (+) and (-) enantiomers of Bay K 8644, at higher concentrations such as 10-4 M, greatly depressed the amplitudes of twitches. Potentiating and depressing effects of (-) enantiomer of Bay K 8644 on twitch responses were significantly greater than those of the (+) enantiomer. At all concentrations used, both (+) and (-) enantiomers of Bay K 8644 significantly decreased the area under the tetanic force × time curve. In intracellular recordings, it was found that the depressing effects of both (+) and (-)-Bay K 8644 on tetanic contractions and twitch responses were due to the inhibition of action potentials. The inhibitory effect of (-) enantiomer of Bay K 8644 on action potentials also was significantly greater than that of the (+) enantiomer. In conclusion, present results suggest that, in contrast with cardiac muscle fibers, (+) and (-) enantiomers of Bay K 8644 have similar inhibitory effects on the electrical and mechanical properties of frog skeletal muscle fibers.Key words: Bay K 8644, calcium channels, sodium channels, skeletal muscle.

Cholecystokinin/opioid interactions

Cholecystokinin (CCK) acts as an anti-opioid peptide. The mechanisms of CCK-opioid interaction under normal and pathological conditions were examined with various techniques. Nerve injury induces upregulation of CCK mRNA and CCK2 receptors in sensory neurons. The involvement of CCK in spinal nociception in normal and axotomized rats was examined. The CCK2 receptor antagonist CI-988 did not reduce spinal hyperexcitability following repetitive C-fiber stimulation in normal or axotomized rats, suggesting that CCK is probably not released from injured primary afferents. With in vivo microdialysis intravenous (i.v.) or intrathecal (i.t.) morphine increased the extracellular level of CCK in the dorsal horn in a naloxone reversible manner. Morphine also released CCK after axotomy, but not during carrageenan-induced inflammation. In contrast, K(+)-stimulation failed to increase extracellular levels of CCK in axotomized rats, but did so in inflamed rats. Double-coloured immunofluorescence technique revealed partial co-localization between CCK-like immunoreactivity (LI) and mu-opioid receptor (MOR)-LI in superficial dorsal horn neurons. The presence of MOR in CCK containing neurons suggests a possible direct influence of opioids on CCK release in the spinal cord. Axotomy, but not inflammation, induced a moderate decrease in CCK- and MOR-LI in the dorsal horn. I.v. morphine further temporarily reduced CCK- and MOR-LIs in axotomized, but not in normal or inflamed, rats. While the effect of morphine on CCK-LI can be interpreted as the result of increased CCK release, the effect on MOR-LI may be related to changes in the microenvironment of the dorsal horn induced by nerve injury.

The effects of endomorphin-1 and endomorphin-2 in CHO cells expressing recombinant mu-opioid receptors and SH-SY5Y cells

1 Endomorphin-1 and -2 (E-1/E-2) have been proposed as endogenous ligands for the mu-opioid receptor. The aims of this study are to characterize the binding of E-1/E-2 and the subsequent effects on cyclic AMP formation and [Ca2+]i levels in SH-SY5Y and Chinese hamster ovary (CHO) cells expressing endogenous and recombinant mu-opioid receptors. 2 E-1 displaced [3H]-diprenorphine ([3H]-DPN) binding in CHO micro and SH-SY5Y membranes with pKi values of 8.02+/-0.09 and 8.54+/-0.13 respectively. E-2 displaced [3H]-DPN binding in CHOmu and SH-SY5Y cells with pKi values of 7.82+/-0.11 and 8.43+/-0.13 respectively. E-1/E-2 bound weakly to CHOdelta and CHOkappa membranes, with IC50 values of greater than 10 microM. 3 In CHOmu cells, E-1/E-2 inhibited forskolin (1 microM) stimulated cyclic AMP formation with pIC50 values of 8.03+/-0.16 (Imax = 53.0+/-9. 3%) and 8.15+/-0.24 (Imax = 56.3+/-3.8%) respectively. In SH-SY5Y cells E1/E2 inhibited forskolin stimulated cyclic AMP formation with pIC50 values of 7.72+/-0.13 (Imax=46.9+/-5.6%) and 8.11+/-0.31 (Imax = 40.2+/-2.8%) respectively. 4 E-1/E-2 (1 microM) increased [Ca2+]i in fura-2 loaded CHOmu cell suspensions in a thapsigargin sensitive and naloxone reversible manner. Mean increases observed were 106+/-28 and 69+/-6.7 nM respectively. In single adherent cells E-1/E-2 (1 microM) increased [Ca2+]i with a mean 340/380 ratio change of 0.81+/-0.09 and 0.40+/-0.08 ratio units respectively. E-1/E-2 failed to increase intracellular calcium in CHOdelta, CHOkappa and SH-SY5Y cells. 5 These data show that E-1/E-2 bind with high affinity and selectivity to mu-opioid receptors and modulate signal transduction pathways typical of opioids. This provides further evidence that these two peptides may be endogenous ligands at the mu-opioid receptor.

The effect of C-terminal truncation of the recombinant delta-opioid receptor on Ca2+i signaling

We have previously shown a stimulatory coupling of the recombinant delta-opioid receptor to phospholipase C leading to production of inositol (1,4,5) triphosphate [Ins(1,4,5)P3] that is affected by truncation of the C-terminus of the receptor. Using a C-terminal mutant of the delta-opioid receptor lacking the final 37 amino acids (CHOdelta37), we examined its coupling to intracellular calcium ion concentration ([Ca2+]i) compared to the full length wild type receptor (CHOdeltaWT) in transfected Chinese hamster ovary (CHO) cells. D-[Pen2,5]enkephalin (DPDPE) mediated increases in [Ca2+]i were measured fluorimetrically in fura-2 loaded whole cell suspensions. DPDPE produced time- and concentration-dependent increases in [Ca2+]i in CHOdeltaWT and CHOdelta37. In both cell types the DPDPE simulated increase in [Ca2+]i was naloxone reversible and pertussis toxin and thapsigargin sensitive. Removal of the C-terminus resulted in a rightward shift of the Ca2+ release concentration-response curve [pEC50 = 8.43 +/- 0.13 and 6.08 +/- 0.25 for CHOdeltaWT and CHOdelta37, respectively]. These data indicate that the C-terminus of the recombinant delta-opioid receptor is important in [Ca2+]i coupling and may be attributed to the effect of C-terminus truncation on phospholipase C coupling reported previously.

Stimulation of mu- and delta-opioid receptors enhances phosphoinositide metabolism in mouse spinal cord: evidence for subtypes of delta-receptors

The accumulation of inositol phosphates (IPs) induced by agonist-activated opioid receptors was analysed in mouse spinal cord slices pre-labelled with myo-[3H]inositol. Agonists showing selectivity to mu-opioid receptors, morphine and [D-Ala2,MePhe4, Gly(ol)5]enkephalin (DAMGO), promoted concentration-dependent increases in the formation of IPs. The activation of delta-opioid receptors by the selective agonists [D-Pen2,5]enkephalin (DPDPE) and [D-Ala2]deltorphin II produced similar increases in phosphoinositide (PI) metabolism. Pre-treatment of the slices with pertussis toxin (PTX) blocked the effect of opioid agonists on IP production. The involvement of Gi/Go-protein (guanine nucleotide-binding protein) classes in this opioid effect is therefore suggested. The activity of the opioid agonists was reduced by the opioid antagonists naltrexone and naloxone. The antagonist at delta1-receptors, 7-benzylidenenaltrexone (BNTX), exhibited greater potency than the antagonists at delta2-receptors, naltriben methanesulphonate (NTB) or naltrindrole 5'-isothiocyanate (NT II), in reducing the activating effect of DPDPE on phosphoinositide metabolism. Conversely, NTB and NT II were more potent antagonists of the activity of [D-Ala2]deltorphin II than BNTX. This work demonstrates the coupling of spinal mu- and delta-opioid receptors to phospholipase C and the generation of IPs. It also provides biochemical evidence for pharmacological subtypes of delta-opioid receptors in the activation of this signalling pathway.

Role of protein kinase C (PKC) in agonist-induced mu-opioid receptor down-regulation: II. Activation and involvement of the alpha, epsilon, and zeta isoforms of PKC

Phosphorylation of specific amino acid residues is believed to be crucial for the agonist-induced regulation of several G protein-coupled receptors. This is especially true for the three types of opioid receptors (mu, delta, and kappa), which contain consensus sites for phosphorylation by numerous protein kinases. Protein kinase C (PKC) has been shown to catalyze the in vitro phosphorylation of mu- and delta-opioid receptors and to potentiate agonist-induced receptor desensitization. In this series of experiments, we continue our investigation of how opioid-activated PKC contributes to homologous receptor down-regulation and then expand our focus to include the exploration of the mechanism(s) by which mu-opioids produce PKC translocation in SH-SY5Y neuroblastoma cells. [D-Ala2,N-Me-Phe4,Gly-ol]enkephalin (DAMGO)-induced PKC translocation follows a time-dependent and biphasic pattern beginning 2 h after opioid addition, when a pronounced translocation of PKC to the plasma membrane occurs. When opioid exposure is lengthened to >12 h, both cytosolic and particulate PKC levels drop significantly below those of control-treated cells in a process we termed "reverse translocation." The opioid receptor antagonist naloxone, the PKC inhibitor chelerythrine, and the L-type calcium channel antagonist nimodipine attenuated opioid-mediated effects on PKC and mu-receptor down-regulation, suggesting that this is a process partially regulated by Ca2+-dependent PKC isoforms. However, chronic exposure to phorbol ester, which depletes the cells of diacylglycerol (DAG) and Ca2+-sensitive PKC isoforms, before DAMGO exposure, had no effect on opioid receptor down-regulation. In addition to expressing conventional (PKC-alpha) and novel (PKC-epsilon) isoforms, SH-SY5Y cells also contain a DAG- and Ca2+-independent, atypical PKC isozyme (PKC-zeta), which does not decrease in expression after prolonged DAMGO or phorbol ester treatment. This led us to investigate whether PKC-zeta is similarly sensitive to activation by mu-opioids. PKC-zeta translocates from the cytosol to the membrane with kinetics similar to those of PKC-alpha and epsilon in response to DAMGO but does not undergo reverse translocation after longer exposure times. Our evidence suggests that direct PKC activation by mu-opioid agonists is involved in the processes that result in mu-receptor down-regulation in human neuroblastoma cells and that conventional, novel, and atypical PKC isozymes are involved.

Autoinhibition, sympathetic cotransmission and biphasic contractile responses to trains of nerve stimulation in the rodent vas deferens

1. The present review critically discusses the evidence for and against the various hypotheses that have been proposed to explain the biphasic contractile response of the rodent vas deferens to trains of electrical field stimulation (EFS). 2. It is widely accepted that the initial component of the biphasic response of the rodent isolated vas deferens to trains of EFS is mediated by ATP and the second slower tonic contractions is mediated by noradrenaline (NA). This theory is based on the ability of antagonists of the post-junctional receptors for these neurotransmitters to inhibit the respective components of the biphasic response and on the ability of exogenous application of either ATP or NA to mimic the responses of each phase. 3. Prejunctional autoinhibition has also been proposed as the cause of the biphasic response. This is based primarily on the ability of alpha 2-adrenoceptor antagonists to transform responses from biphasic to monophasic and on the ability of neuronal NA uptake inhibitors to accentuate the separation of the two phases. 4. Atypical or extrajunctional NA receptors have also been proposed to be the mediators of the component of the response to nerve stimulation that is resistant to the traditional alpha-adrenoceptor antagonists. 5. Different contractile mechanisms and/or sources of calcium have also been postulated to cause the biphasic response. Blockers of intracellular Ca2+ mobilization are able to block the initial component, while blockers of extracellular Ca2+ entry inhibit the second tonic phase. 6. It is concluded that because alpha 1-adrenoceptor antagonists and blockers of P2 purinoceptors have also been shown to block both phases of the response to trains of EFS, prejunctional auto-inhibitory mechanisms perhaps provide the most sound explanation for the phenomenon of the biphasic contractile response to trains of EFS.

Delta-opioid receptors on astroglial cells in primary culture: mobilization of intracellular free calcium via a pertussis sensitive G protein

Astrocytes in primary culture from rat cerebral cortex were probed concerning the expression of delta-opioid receptors and their coupling to changes in intracellular free calcium concentrations ([Ca2+]i). Fluo-3 or fura-2 based microspectrofluorometry was used for [Ca2+]i measurements on single astrocytes in a mixed astroglial-neuronal culture. Application of the selective delta-opioid receptor agonist, [D-Pen2, D-Pen5]-enkephalin (DPDPE), at concentrations ranging from 10 nM to 100 microM, induced concentration-dependent increases in [Ca2+]i (EC50 = 114 nM). The responses could be divided into two phases, with an initial spike in [Ca2+]i followed by either oscillations or a sustained elevation of [Ca2+]i. These effects were blocked by the selective delta-opioid receptor antagonist ICI 174864 (10 microM). The expression of delta-opioid receptors on astroglial cells was further verified immunohistochemically, using specific antibodies, and by Western blot analyses. Pre-treatment of the cells with pertussis toxin (100 ng/ml, 24 h) blocked the effects of delta-opioid receptor activation, consistent with a Gi- or Go-mediated response. The sustained elevation of [Ca2+]i was not observed in low extracellular Ca2+ and was partly blocked by nifedipine (1 microM), indicating the involvement of L-type Ca2+ channels. Stimulating neurons with DPDPE resulted in a decrease in [Ca2+]i, which may be consistent with the closure of the plasma membrane Ca2+ channels on these cells. The current results suggest a role for astrocytes in the response of the brain to delta-opioid peptides and that these opioid effects in part involve altered astrocytic intracellular Ca2+ homeostasis.

Inhibition of phospholipase C blocks opioid receptor-mediated activation of Gi proteins

Activation of phospholipase C (PLC) in response to stimulation of delta-opioid, m4 muscarinic and alpha2 adrenergic receptors was observed in NG108-15 cells. Treatment with PLC specific inhibitors, U73122 and ET-18-OCH3, blocked delta-opioid receptor-mediated activation of G proteins with no effect on opioid binding to the receptors. U73122 treatment also suppressed functional responses of m4 muscarinic and alpha2 adrenergic receptors in NG108-15 cells. Furthermore, the G protein activation mediated by mu- and delta-opioid receptors and opioid receptor-like receptor (ORL1) were abolished by U73122 in SK-N-SH neuroblastoma cells. Inhibition of adenylyl cyclase induced by high concentrations of GTP was blocked by U73122, suggesting that blockade is at the level of G proteins. Our results thus indicated that inhibition of PLC leads to blockade of Gi protein activation mediated by opioid receptors or other Gi-coupled receptors.

Frequency-dependent effects of Bay K 8644 on tetanic contractions of frog skeletal muscle

1. The effects of the calcium channel agonist, Bay K 8644 (1-100 microM), on tetanic contractions, elicited by the stimulation frequencies of 100 Hz, 50 Hz and 25 Hz for 2 s, were investigated on frog skeletal muscle fibers. 2. Although the area under the tetanic force versus time curve was greatly reduced at the stimulation frequency of 100 Hz, this effect was significantly reversed at the lower stimulation frequencies of 50 and 25 Hz, at all concentrations tested. 3. During the intracellular recordings, it was revealed that the sodium action potentials elicited with the stimulation frequency of 100 Hz for 2 s were significantly blocked. 4. Similar to mechanical recordings, the blockade of repetitively elicited action potentials was also significantly reversed at lower stimulation frequencies of 50 and 25 Hz for all concentrations of Bay K 8644 tested. 5. In conclusion, the results indicate that Bay K 8644 depresses both tetanic contractions and action potentials in a frequency-dependent manner in frog skeletal muscle fibers.

Effect on the indo-1 transient of applying Ca2+ channel blocker for a single beat in voltage-clamped guinea-pig cardiac myocytes

1. We used rapid solution changes to investigate the mechanisms which trigger Ca2+ release from the sarcoplasmic reticulum (SR) in guinea-pig ventricular myocytes. We patch-clamped myocytes at 36 degrees C and used indo-1 to monitor intracellular Ca2+. Before each test pulse, we established a standard level of SR Ca2+ load by applying a train of conditioning pulses. 2. We switched rapidly to 32 microM nifedipine (an L-type Ca2+ current (ICa,L) blocker) 8 s before a test pulse, and just after applying nifedipine we applied a ramp depolarization to pre-block Ca2+ channels. We found that ICa,L elicited by the following test pulse was inhibited almost completely (98-99% inhibition). 3. The indo-1 transient elicited by an 800 ms depolarizing pulse showed a rapid initial rise which was inhibited by ryanodine-thapsigargin. This indicated that the rapid rise was due to Ca2+ release from the SR, and therefore provides an index of SR Ca2+ release. 4. In cells dialysed internally with 10 mM Na(+)-containing solution, nifedipine application before a +10 mV test pulse blocked 62% of the rapid initial phase of the indo-1 transient. Calibration curves of indo-1 for intracellular Ca2+ (using a KD of indo-1 for Ca2+ of either 250 or 850 nM, the reported range) indicated that between 67 and 76% of the Ca2+i transient was inhibited by nifedipine. Thus, in cells dialysed with 10 mM Na+ and depolarized to +10 mV, and in the absence of ICa,L, this suggests that another trigger mechanism for SR release is able to trigger between 33 and 24% of the Ca2+i transient. 5. For a given dialysing Na+ concentration, the fraction of indo-1 transient which was inhibited by nifedipine decreased as test potential became more positive. In cells dialysed with 10 mM Na+ and pulsed to +110 mV, 24% of the rapid phase of the indo-1 transient was inhibited by nifedipine (equivalent to between 27 and 37% of the Ca2+i transient). 6. For a given test potential, the fraction of the indo-1 transient which was inhibited by nifedipine decreased as dialysing Na+ concentration increased. In cells dialysed with Na(+)-free solution and pulsed to +10 mV, 84% of the indo-1 transient was inhibited by nifedipine (equivalent to between 88 and 91% of the Ca2+i transient). In contrast, in cells dialysed with 20 mM Na+ and pulsed to +10 mV, 41% of the indo-1 transient was inhibited by nifedipine (equivalent to between 47 and 57% of the Ca2+i transient). 7. Dialysing cells with different Na+ concentrations could lead to a different SR Ca2+ content. We therefore manipulated the conditioning train before each test pulse to change the extent of SR loading. For each dialysing Na+ concentration, we found no change in the degree to which nifedipine blocked the indo-1 transient when SR content was either increased or decreased. 8. The results support the idea that both ICa, L and a second mechanism are able to trigger SR release and the resulting Ca2+i transient. When ICa, L was blocked with nifedipine, the fraction of Ca2+i transient which remained increased with more positive test potential and higher internal Na+. This is consistent with the hypothesis that the second SR trigger mechanism is Ca2+ entry via reverse Na(+)-Ca2+ exchange, elicited by a step change in membrane potential.

Silent calcium channels generate excessive tail currents and facilitation of calcium currents in rat skeletal myoballs

1. Whole-cell patch-clamp recording were employed to study facilitation of Ca2+ currents and excessive Ca2+ tail currents evoked by strong and long-lasting conditioning depolarizations in skeletal myoballs cultured from newborn rats. 2. Paired-pulse facilitation and excessive tail currents showed the same voltage dependence, becoming prominent at conditioning potentials above +30 mV. 3. Recruitment of excessive tail currents and facilitation occurred with the same time dependence (time constant (tau), approximately 200 ms to approximately 1 s), accelerating with the depolarization strength of conditioning pulses. 4. Reversal of Ca2+ current facilitation during the repolarization period between conditioning and test pulses was time- and voltage dependent. The time window of recruitment of facilitated Ca2+ currents narrowed considerably at more negative repolarization potentials (tau: approximately 10 ms at -100 mV, but approximately 1.5 at 0 mV). 5. Neither omission of internal ATP nor perfusion of the cells with the peptide inhibitor of protein kinase A (PKI) had significant effects on Ca2+ current facilitation, although internal perfusion with ATP gamma S slowly suppressed the facilitation currents by about 30%. External application of either ryanodine or caffeine under control conditions selectively and significantly suppressed the facilitated Ca2+ currents by about 30-40%. 6. We propose that facilitation of Ca2+ currents and excessive tail currents are consequences of a common mechanism linked to ryanodine receptors.

Binding constants determined from Ca2+ current responses to rapid applications and washouts of nifedipine in frog cardiac myocytes

1. A fast perfusion system was used to analyse the kinetics of the response of L-type calcium current (ICa) to rapid applications and washouts of the dihydropyridine antagonist nifedipine in whole-cell patch-clamped frog ventricular myocytes. 2. Both the inhibition of ICa induced by nifedipine and the recovery from inhibition upon washout of the drug behaved as mono-exponential functions of time. 3. During application or washout of 100 nM nifedipine, only the peak amplitude of ICa varied but not its time course of activation or inactivation. 4. The rate constant of the onset of ICa inhibition increased with the concentration of nifedipine. However, the time course of the recovery from inhibition was independent of drug concentration. 5. Both rate constants were strongly sensitive to the holding potential but insensitive to the test potential. 6. Using simple rate equations and a one-binding-site analysis it was possible to determine the rate constants for association (k1) and dissociation (k-1) and the equilibrium dissociation constant (KD) of the reaction between nifedipine and Ca2+ channels. KD values for nifedipine were identical to IC50 values obtained from classical steady-state experiments. 7. With depolarized holding potentials, KD decreased strongly due to a large reduction in k-1 and a modest increase in k1. Assuming that these changes result from the distribution of Ca2+ channels between resting and inactivated states, a low-affinity binding to the resting state (R) and a high-affinity binding to the inactivated state (I) were obtained with the binding constants: k1R = 1.0 x 10(6) M-1 S-1, k-1R = 0.077 S-1, and KDR = 77 nM for the resting state; k1I = 4.47 x 10(6) M-1 S-1, k-1I = 7.7 x 10(-4) S-1, and KDI = 0.17 nM for the inactivated state. 8. Rapid application/washout experiments provide a unique way to determine, in an intact cell and in a relatively short period (2-4 min), the binding rate constants and the KD value of the reaction between a dihydropyridine antagonist and the Ca2+ channels.

Effects of calcium channel modulators on the regulation of cytoplasmic Ca2+ and hormone secretion of parathyroid cells

Effects of Ca(2+)-channel modulators were examined in human, bovine and murine parathyroid tissue. In 0.5 mM external Ca2+, 100 microM verapamil inhibited parathyroid hormone release, stimulated uptake and efflux of 45Ca and raised cytoplasmic Ca2+ concentration ([Ca2+]i). However, in 3.0 mM Ca2+ the drug only affected efflux. Methoxyverapamil (50 microM) inhibited parathyroid hormone release in 0.5 mM but stimulated secretion in 3.0 mM Ca2+. BAY K 8644 (10 microM) had similar actions as verapamil on parathyroid hormone release and [Ca2+]i, whereas nifedipine (1 microM) and diltiazem (50-100 microM) lacked effects. Despite the lack of voltage-dependent Ca2+ channels in parathyroid cells, drugs with established actions on such channels affect [Ca2+]i and parathyroid hormone release. However, these actions are not sufficiently pronounced and tissue specific to allow their use for correcting hyperparathyroidism.

Long-term depression of a sympathetic ganglionic response to opioids by prolonged synaptic activity and by phorbol esters

We studied the effect of the adenylate cyclase activator forskolin, of protein kinase C-activating phorbol esters and of prolonged preganglionic input activation on the inhibitory response of the perfused superior cervical ganglion of the cat to exogenous met-enkephalin (Met-ENK). Met-ENK inhibited, in a concentration-dependent manner, the postganglionic compound action potential evoked by cervical sympathetic trunk stimulation. The inhibition was reversible, was blocked by naloxone as well as by pertussis toxin and showed no homologous desensitization in the concentration range 0.01-10 microM. Pretreatment of the ganglion with 4 beta-phorbol 12,13-dibutyrate or 4 beta-phorbol 12,13-diacetate depressed the Met-ENK response for several hours, while pretreatment with forskolin had no effect. This action of phorbol esters was prevented by the protein kinase inhibitor H-7 but not by the calmodulin antagonist W-7 or the protein kinase A inhibitor HA 1004 and was calcium-dependent. Recovery of the response from the depression produced by phorbol esters was not affected by a protein synthesis inhibitor. A 40 Hz 20 min stimulus train to the cervical sympathetic trunk mimicked the effect of phorbol esters, depressing for several hours the inhibition produced by Met-ENK. Stimulus trains of duration shorter than 5 min or frequency lower than 5 Hz were ineffective. This effect of prolonged preganglionic stimulation occurred even when the stimulus train was delivered during complete block of nicotinic and muscarinic ganglionic transmission but was lost when the stimulus train was delivered during perfusion with calcium-free Krebs. The protein kinase inhibitor H-7 prevented the depression of the Met-ENK response by the train, while W-7 and HA 1004 had no effect. These findings suggest that, in the superior cervical ganglion of the cat, a kinase, activated by phorbol esters and inhibited by H-7, exerts a long-term control of the ganglion cell responsiveness to opiate receptor activation. A similar mechanism can be synaptically activated by a non-cholinergic transmitter, released by the preganglionic axons during prolonged, high frequency, activity.

Desensitization of the mu-opioid activation of phospholipase C in SH-SY5Y cells: the role of protein kinases C and A and Ca(2+)-activated K+ currents

1. In SH-SY5Y cells, mu-opioids cause a rapidly desensitizing activation of phospholipase C (PLC), that appears secondary to Ca2+ influx via L-type voltage-sensitive Ca2+ channels (VSCCs). The aim of the present study was to characterize the mechanisms of desensitization of the mu-opioid-induced inositol (1,4,5) triphosphate (Ins(1,4,5)P3) response, by use of a stereospecific radioreceptor mass assay. 2. (R+)-Bay K 8644 (1 nM-10 microM) dose-dependently inhibited fentanyl-induced Ins(1,4,5)P3 formation, with an IC50 of 28.5 nM, confirming our earlier observations that mu-opioids open L-type VSCCs, thus allowing Ca2+ influx to activate PLC. 3. Ro 31-8220 (0.1 nM-10 microM), a protein kinase C inhibitor, dose-dependently enhanced fentanyl-induced Ins(1,4,5)P3 formation (EC50 = 20.0 nM), whilst acute phorbol 12,13-dibutrate (1 microM) abolished the response. 4. H-89 (1 nM-10 microM), a protein kinase A inhibitor, also dose-dependently enhanced fentanyl-induced Ins(1,4,5)P3 formation (EC50 = 93 nM), whilst dibutryl cyclic AMP (0.5 mM) abolished the response. 5. Blockade of Ca(2+)-activated K+ currents with 4-aminopyridine (2 mM) or iberiotoxin (10 nM) had no effect on fentanyl-induced Ins(1,4,5)P3 formation but further increased the Ro 31-8220-enhanced response. 6. All three mechanisms had additive, or even supra-additive, effects, but only at later (120-300 s) time points. In addition, fentanyl-induced Ins(1,4,5)P3 formation, even if enhanced by H-89, Ro 31-8220 and/or 4-aminopyridine, was inhibited by nifedipine (1 nM-10 microM). 7. In conclusion, desensitization of the mu-opioid-induced activation of PLC is multifactorial, involving protein kinases C and A and Ca(2+)-activated K+ efflux, but the L-type VSCC is of critical importance and may be a possible common site of action.

Mu-opioids activate phospholipase C in SH-SY5Y human neuroblastoma cells via calcium-channel opening

We have recently reported that, in SH-SY5Y cells, mu-opioid receptor occupancy activates phospholipase C via a pertussis toxin-sensitive G-protein. In the present study we have further characterized the mechanisms involved in this process. Fentanyl (0.1 microM) caused a monophasic increase in inositol 1,4,5-trisphosphate mass formation, with a peak (20.5 +/- 3.6 pmol/mg of protein) at 15 s. Incubation in Ca(2+)-free buffer abolished this response, while Ca2+ replacement 1 min later restored the stimulation of inositol 1,4,5-trisphosphate formation (20.1 +/- 0.6 pmol/mg of protein). In addition, nifedipine (1 nM-0.1 mM), an L-type Ca(2+)-channel antagonist, caused a dose-dependent inhibition of inositol 1,4,5-trisphosphate formation, with an IC50 of 60.3 +/- 1.1 nM. Elevation of endogenous beta/gamma subunits by selective activation of delta-opioid and alpha 2 adrenoceptors failed to stimulate phospholipase C. Fentanyl also caused a dose-dependent (EC50 of 16.2 +/- 1.0 nM), additive enhancement of carbachol-induced inositol 1,4,5-trisphosphate formation. In summary, we have demonstrated that in SH-SY5Y cells activation of the mu-opioid receptor allows Ca2+ influx to activate phospholipase C. However, the possible role of this mechanism in the process of analgesia remains to be elucidated.

Calcium channels in cerebral arteries

Electrophysiological evidence shows the existence of voltage-operated Ca2+ channels of the L- and, in some cases, T- and B-, type in the smooth muscle cells of major cerebral arteries and arterioles. Current intensity through L-type Ca2+ channels is higher in cerebral than in peripheral arteries, which points to a greater dependence on extracellular Ca2+ of contractile responses in cerebral arteries. The increase in cytosolic Ca2+ concentration is the key event leading both to maintenance of basal cerebrovascular tone and to contraction of cerebral arteries in response to depolarization and agonist-receptor interaction. Such an increase results from increased transmembrane influx of Ca2+ through L-type Ca2+ channels, as well as from the release of Ca2+ from intracellular Ca2+ stores. Ca2+ entry modulators (dihydropyridines, phenylalkylamines, benzothiazepines, and diphenylpiperazines) bind to allosterically coupled sites in the Ca2+ channel, thus inhibiting (Ca2+ entry blockers) or stimulating (Ca2+ entry activators) Ca2+ influx and, therefore, contractile responses of the cerebral arteries. In vivo, Ca2+ entry blockers increase pial vascular caliber and cerebral blood flow by their direct action on the cerebroarterial wall. However, such an action also takes place on several peripheral vascular beds, which leads to hypotension. Therefore, the brain cannot be considered a "privileged" organ when the vasodilatatory action of Ca2+ entry blockers is considered. Since increased cytosolic Ca2+ concentration (and, therefore, activation of Ca2+ channels) plays a crucial role in the pathogenesis of ischemic brain damage (e.g., acute stroke and subarachnoid hemorrhage), Ca2+ entry blockers could be useful cytoprotective drugs. However, with the exception of nimodipine in the management of subarachnoid hemorrhage, clinical trials have yielded results that are not so promising as one could expect from those obtained in experimental research.

Effect of continuous intrathecal infusion of omega-conopeptides, N-type calcium-channel blockers, on behavior and antinociception in the formalin and hot-plate tests in rats

The effect of continuous intrathecal infusion of omega-conopeptides in the rat was examined to determine whether antinociception, as measured on the formalin and hot-plate (52.5 degrees C) tests, was altered and whether tolerance developed with chronic infusion of these agents. Infusion of 0.030 and 0.003 nmol/h SNX-111 and 0.290 nmol/h SNX-239 was performed for either 2 days ('acute') or 7 days ('chronic') and was compared to the effect of 20 nmol/h morphine or saline. Both doses of SNX-111 and SNX-239 produced a significant reduction of the response to the hot-plate and formalin tests at both 2 and 7 days of infusion compared to saline infusion. In contrast, morphine only produced a significant effect on day 2, but not on infusion day 7, indicating that tolerance had developed. The effect of SNX-111 was reversible, as shown by a return to nociceptive responses similar to saline-infused rats 2 days after the minipumps had been disconnected after a 7-day infusion period. These data indicate that chronic infusion of omega-conopeptides that block N-type voltage-sensitive calcium channels produce a powerful antinociception, with minimal development of tolerance.

Oxygen free radicals and calcium homeostasis in the heart

Many experiments have been done to clarify the effects of oxygen free radicals on Ca2+ homeostasis in the hearts. A burst of oxygen free radicals occurs immediately after reperfusion, but we have to be reminded that the exact levels of oxygen free radicals in the hearts are yet unknown in both physiological and pathophysiological conditions. Therefore, we should give careful consideration to this point when we perform the experiments and analyze the results. It is, however, evident that Ca2+ overload occurs when the hearts are exposed to an excess amount of oxygen free radicals. Through ATP-independent Ca2+ binding is increased, Ca2+ influx through Ca2+ channel does not increase in the presence of oxygen free radicals. Another possible pathway through which Ca2+ can enter the myocytes is Na(+)-Ca2+ exchanger. Although, the activities of Na(+)-K+ ATPase and Na(+)-H(+) exchange are inhibited by oxygen free radicals, it is not known whether intracellular Na(+) level increases under oxidative stress or not. The question has to be solved for the understanding of the importance of Na(+)-Ca2+ exchange in Ca2+ influx process from extracellular space. Another question is 'which way does Na(+)-Ca2+ exchange work under oxidative stress? Net influx or efflux of Ca2+?' Membrane permeability for Ca2+ may be maintained in a relatively early phase of free radical injury. Since sarcolemmal Ca(2+)-pump ATPase activity is depressed by oxygen free radicals, Ca2+ extrusion from cytosol to extracellular space is considered to be reduced. It has also been shown that oxygen free radicals promote Ca2+ release from sarcoplasmic reticulum and inhibit Ca2+ sequestration to sarcoplasmic reticulum. Thus, these changes in Ca2+ handling systems could cause the Ca2+ overload due to oxygen free radicals.

Oxygen free radicals and calcium homeostasis in the heart

Many experiments have been done to clarify the effects of oxygen free radicals on Ca2+ homeostasis in the hearts. A burst of oxygen free radicals occurs immediately after reperfusion, but we have to be reminded that the exact levels of oxygen free radicals in the hearts are yet unknown in both physiological and pathophysiological conditions. Therefore, we should give careful consideration to this point when we perform the experiments and analayze the results. It is, however, evident that Ca2+ overload occurs when the hearts are exposed to an excess amount of oxygen free radicals. Though ATP-independent Ca2+ binding is increased, Ca2+ influx through Ca2+ channel does not increase in the presence of oxygen free radicals. Another possible pathway through which Ca2+ can enter the myocytes is Na(+)-Ca2+ exchanger. Although, the activities of Na(+)-K+ ATPase and Na(+)-Ca2+ exchanger. Although, the activities of Na(+)-H+ exchange are inhibited by oxygen free radicals, it is not known whether intracellular Na+ level increases under oxidative stress or not. The question has to be solved for the understanding of the importance of Na(+)-Ca2+ exchange in Ca2+ influx process from extracellular space. Another question is 'which way does Na(+)-Ca2+ exchange work under oxidative stress? Net influx or efflux of Ca2+?' Membrane permeability for Ca2+ may be maintained in a relatively early phase of free radical injury. Since sarcolemmal Ca(2+)-pump ATPase activity is depressed by oxygen free radicals, Ca2+ extrusion from cytosol to extracellular space is considered to be reduced. It has also been shown that oxygen free radicals promote Ca2+ release from sarcoplasmic reticulum and inhibit Ca2+ sequestration to sarcoplasmic reticulum. Thus, these changes in Ca2+ handling systems could cause the Ca2+ overload due to oxygen free radicals.

Cardiac T-type calcium current: pharmacology and roles in cardiac tissues

A low threshold, voltage-gated calcium current is reported in most cardiac tissues but rarely in ventricular cells. This article reports some recently described characteristics and discusses their possible pathophysiologic implications. It also reviews the alterations induced in this current by a variety of chemical agents including several neuromediators in cardiac and other tissues.

mu-Opioid receptor stimulation of inositol (1,4,5)trisphosphate formation via a pertussis toxin-sensitive G protein

The cellular mechanisms underlying opioid action remain to be fully determined, although there is now growing indirect evidence that some opioid receptors may be coupled to phospholipase C. Using SH-SY5Y human neuroblastoma cells (expressing both mu- and delta-opioid receptors), we demonstrated that fentanyl, a mu-preferring opioid, caused a dose-dependent (EC50 = 16 nM) monophasic increase in inositol (1,4,5)trisphosphate mass formation that peaked at 15 s and returned to basal within 1-2 min. This response was of similar magnitude (25.4 +/- 0.8 pmol/mg of protein for 0.1 microM fentanyl) to that found in the plateau phase (5 min) following stimulation with 1 mM carbachol (18.3 +/- 1.4 pmol/mg of protein), and was naloxone-, but not naltrindole- (a delta antagonist), reversible. Further studies using [D-Ala2, MePhe4, Gly(ol)5]enkephalin and [D-Pen2,5]enkephalin confirmed that the response was specific for the mu receptor. Incubation with Ni2+ (2.5 mM) or in Ca(2+)-free buffer abolished the response, as did pretreatment (100 ng/ml for 24 h) with pertussis toxin (control plus 0.1 microM fentanyl, 26.9 +/- 1.5 pmol/mg of protein; pertussis-treated plus 0.1 microM fentanyl, 5.1 +/- 1.3 pmol/mg of protein). In summary, we have demonstrated a mu-opioid receptor-mediated activation of phospholipase C, via a pertussis toxin-sensitive G protein, that is Ca(2+)-dependent. This stimulatory effect of opioids on phospholipase C, and the potential inositol (1,4,5)trisphosphate-mediated rises in intracellular Ca2+, could play a part in the cellular mechanisms of opioid action.

Alpha 1-adrenoceptors and calcium sources in adrenergic neurogenic contractions of rat vas deferens

1. The involvement of alpha 1-adrenoceptor subtypes in adrenergic neurogenic contractions of different type was studied in epididymal and prostatic portions of the rat vas deferens. 2. The adrenergic component of neurogenic contractions was isolated by suramin (300 microM). Twitch-like and tonic contractions were elicited by appropriate pulse patterns of electrical field stimulation, and contractions relying on intracellular calcium mobilization and calcium entry were isolated by means of nifedipine (10 microM) and ryanodine (20 microM), respectively. Increasing concentrations of 2-(2,6-dimethoxyphenoxyethyl)aminomethyl-1,4-benzodioxane (WB 4101), alpha-ethyl-3,4,5-trimethoxy-alpha-(3-((2-(2-methoxyphenoxy)ethyl)- amino)-propyl)benzeneacetonitrile (HV 723), prazosin and 5-methylurapidil progressively, monophasically and with potency decreasing in that order reduced and finally abolished all types of contraction, with one exception: concentration-effect curves of 5-methylurapidil in epididymal segments in the presence of ryanodine levelled off at about 75% inhibition. In the presence of both nifedipine (10 microM) and ryanodine (20 microM), contractions were abolished. 3. Contractions elicited by exogenous noradrenaline were also studied in the presence of either nifedipine 10 microM (prostatic segments) or ryanodine 20 microM (epididymal segments). Increasing concentrations of tamsulosin, WB 4101, benoxathian, HV 723, prazosin, 5-methylurapidil and urapidil progressively, monophasically and with potency decreasing in that order reduced and eventually abolished both kinds of contraction, with two exceptions: in epididymal segments in the presence of ryanodine, the concentration-effect curve of 5-methylurapidil was biphasic and the curve of urapidil levelled off at only partial inhibition. 4. In slices prepared from the prostatic end and preincubated with [3H]-noradrenaline, WB 4101, HV 723, prazosin and 5-methylurapidil, at the highest concentrations tested against neurogenic contractions, increased only slightly the overflow of tritium elicited by trains of 50 pulses at 5 Hz. 5. It is concluded that two alpha l-adrenoceptor subtypes mediate adrenergic neurogenic contractions of rat vas deferens. The main one, pharmacologically alpha 1A, activates both calcium mobilization and entry. In addition there is a second receptor, not previously detected in the vas deferens and not corresponding to any named alpha l subtype, characterized by high and similar affinity for tamsulosin, WB 4101, benoxathian,HV 723 and prazosin and very low affinity for 5-methylurapidil and urapidil, and linked exclusively to calcium entry. Both subtypes and their respective transduction pathways also contribute to contractions elicited by exogenous noradrenaline. An alpha 1B-adrenoceptor-mediated contraction was not found under any experimental conditions.

Complement-induced Ca2+ influx in cultured fibroblasts is decreased by the calcium-channel antagonist nifedipine or by some bivalent inorganic cations

The effects of different extracellular cations or organic Ca(2+)-channel modulators on complement-induced changes in intracellular Ca2+ and cell death have been investigated in the transfected NIH-3T3 HIR 3.5 cell line, which overexpresses the human insulin receptor. Cells were incubated with mouse anti-(human insulin receptor) monoclonal antibodies before exposure to rabbit or human serum (sources of heterologous complement). Changes in intracellular Ca2+ were complement-dependent (measured by influx of 45Ca), as was cytotoxicity (monitored by leakage of lactate dehydrogenase into the culture supernatant). Addition of a dihydropyridine Ca(2+)-channel antagonist (nifedipine) or some bivalent inorganic cations caused inhibition of 45Ca entry via a novel channel distinct from endogenous voltage-gated Ca2+ channels. Nifedipine decreased, but conversely the addition of a phenylalkylamine Ca(2+)-channel antagonist (verapamil) or the inorganic Ca2+ agonists Ba2+ and Sr+ increased, complement-induced cytotoxicity. These agents had no effect on cell viability at the studied concentrations, in the absence of complement. It is concluded that complement-induced cytotoxicity is mediated by Ca2+ influx through novel specific transmembrane channels which are sensitive to the Ca(2+)-channel antagonist nifedipine, but otherwise show little resemblance to L- or T-type voltage-gated Ca2+ channels.

Effects of morphine and its metabolites on opiate receptor binding, cAMP formation and [3H]noradrenaline release from SH-SY5Y cells

Opiate receptor occupation leads to a variety of intracellular events including inhibition of adenylyl cyclase and cAMP formation. We have examined the opiate binding characteristics, effects on cAMP formation and [3H]noradrenaline release of morphine, morphine-6 (M6G) and -3 (M3G)-glucuronides, and fentanyl in SH-SY5Y human neuroblastoma cells. M6G and M3G are the major metabolites of morphine formed in vivo whose cellular action remains to be fully elucidated. In binding experiments morphine (affinity, K50 = 96 nM) and fentanyl (K50 = 99 nM) were more potent than M6G (K50 = 393 nM), while M3G was inactive. However, for cAMP inhibition morphine (half maximum inhibition, IC50 = 193 nM) and M6G (IC50 = 113 nM) were roughly equipotent, with fentanyl (IC50 = 27 nM) being more potent and producing a greater maximum inhibition (56%). M3G was inactive. These in vitro data are in general agreement with the in vivo effects of these glucuronides. Moreover, all of the opiates tested failed to inhibit K(+)-evoked release of [3H]noradrenaline. Whilst these data do not support a role for cAMP in neurotransmitter release, alterations in cAMP formation may still have a role to play in the mechanism of analgesia.

Inhibition by ethanol of contractions of rat vas deferens: no evidence for selective blockade of P2X-purinoceptors

Some ligand-gated ion channels are important sites of action of ethanol. The aim of the study was to find out whether the P2X-purinoceptors mediating contraction of the rat isolated vas deferens also are selectively sensitive to ethanol. Contractions were elicited by ATP (1 mmol/l), alpha, beta-methylene ATP (0.3 mumol/l), noradrenaline (3 mumol/l), high K+ (20 mmol/l) or electrical (neural) stimulation by pairs of pulses 3 s apart. In electrical stimulation experiments, purinergic and adrenergic response components were isolated by prazosin and suramin, respectively. Concentration-effect curves were determined for ethanol and, for comparison, nifedipine. Tritium outflow from tissues preincubated with 3H-noradrenaline was also examined. Ethanol at relatively low concentrations reduced contractions elicited by high K+ (IC30 145 mmol/l), ATP (IC30 211 mmol/l) and alpha, beta-methylene ATP (IC30 215 mmol/l) as well the purinergic component of neurogenic twitches (IC30 110-126 mmol/l; a significant effect at 10-32 mmol/l) and the adrenergic component of twitch 2 of the twitch pairs (IC30 63 mmol/l). These contractions also were very sensitive to nifedipine. Higher concentrations of ethanol were needed to reduce contractions elicited by noradrenaline (IC30 365 mmol/l) and the adrenergic component of twitch 1 of the twitch pairs (IC30 382 mmol/l), contractions that also were less sensitive to nifedipine. Ethanol 1 mol/l abolished all contractions. In contrast, concentration-effect curves for the inhibition by nifedipine of contractions evoked by ATP, alpha,beta-methylene ATP and noradrenaline (rapid phase) levelled off at 60-70% inhibition.(ABSTRACT TRUNCATED AT 250 WORDS)

Transmitter release and calcium currents at an Aplysia buccal ganglion synapse--I. Characterization

The Ca2+ current recorded in the presynaptic neuron (B4/B5) of an identified Aplysia synapse was characterized in terms of its activation, voltage sensitivity, Ca2+ dependence of inactivation and pharmacology. It was compared to that recorded in left upper quadrant abdominal ganglion neurons which, unlike B4/B5, display Ca2+ action potentials. The two Ca2+ currents could not be distinguished in terms of their activation threshold or voltage sensitivity. The Ca2+ current recorded in left upper quadrant neurons, however, displayed more important Ca(2+)-dependent inactivation. The peak Ca2+ current in B4/B5 neurons was significantly reduced (30-40%) by the dihydropyridine Ca2+ channel antagonist, nifedipine, while it was increased (15-20%) by the dihydropyridine Ca2+ channel agonist, BAY K8644, although none of these agents had any effect on transmitter release from B4/B5. omega-Conotoxin similarly reduced the Ca2+ current by 30-40%, but unlike nifedipine, it also caused a 50-60% reduction in B4/B5 transmitter release. The pharmacological properties of the Ca2+ current present in left upper quadrant neurons were somewhat different, as this current was unaffected by either BAY K8644 or omega-conotoxin and moderately suppressed (20%) by nifedipine.

Regulation by protein kinase-C of putative P-type Ca channels expressed in Xenopus oocytes from cerebellar mRNA

Xenopus oocytes injected with rat cerebellar mRNA expressed functional voltage-dependent Ca channels detected as an inward Ba current (IBa). The pharmacological resistance to dihydropyridines and omega-conotoxin together with the blockade obtained with Agelenopsis aperta venom suggest that these channels could be somehow assimilated to P-type Ca channels. The precise nature of the transplanted Ca channels was assessed by hybrid-arrest experiments using a specific oligonucleotide antisense-derivated from the recently cloned alpha 1-subunit of P channels (BI-1 clone). In addition, we demonstrate that exogenous Ca channel activity was enhanced by two different PKC activators (a phorbol ester and a structural analog to diacylglycerol). The general electrophysiological and pharmacological properties of the stimulated Ca channels remain unchanged. This potentiation induced by PKC activators is antagonized by a PKC inhibitor (staurosporine) and by a monoclonal antibody directed against PKC. It is concluded that P-type Ca channels are potentially regulated by PKC phosphorylation and the functional relevance of this intracellular pathway is discussed.

Ionic channel currents in cultured neurons from human cortex

Ionic channels in human cortical neurons have not been studied extensively. HCN-1 and HCN-1A cells, which recently were established as continuous cultures from human cortical tissue, have been shown by histochemical and immunochemical methods to exhibit a neuronal phenotype, but expression of functional ionic channels was not demonstrated. For the present study, HCN-1 and HCN-1A cells were cultured in Dulbecco's modified Eagle's medium with 15% fetal calf serum, in some cases supplemented with 10 ng/ml nerve growth factor, 10 microM forskolin, and 1 mM dibutyryl cyclic adenosine monophosphate to promote differentiation. Cells or membrane patches were voltage clamped using conventional patch clamp techniques. In HCN-1A cells, we identified a tetrodotoxin-sensitive Na+ current, two types of Ca2+ channel current, including L-type current and a second type that in some respects resembled N-type current, and four types of K+ current, including a delayed outward rectifier that showed voltage-dependent inactivation, two types of noninactivating Ca(2+)-activated K+ channels with slope conductances of 146 and 23 pS (K+i/K+o 145 mM/5 mM), and less frequently, a noninactivating, intermediate conductance channel that was not sensitive to internal Ca2+. When HCN-1A cells were examined after 3 days of exposure to differentiating agents, pronounced morphological changes were evident but no differences in ionic currents were apparent. HCN-1 cells also exhibited K+ and Ca2+ channel currents, but Na+ currents were not detected in these cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Dihydropyridine binding and Ca(2+)-channel characterization in clonal calcitonin-secreting cells

1,4-Dihydropyridine-sensitive voltage-dependent Ca2+ channels play a crucial role in the extracellular Ca(2+)-sensing of calcitonin-secreting parafollicular cells of the thyroid (C-cells). To characterize the Ca2+ channels in C-cells, we studied 1,4-dihydropyridine binding and performed electrophysiological experiments with Ca(2+)-sensitive C-cells (rat C-cell line rMTC 44-2) in comparison with 'defective' Ca(2+)-insensitive C-cells (human C-cell line TT). In membranes of rMTC cells, we detected a high-affinity, stereoselective and Ca(2+)-dependent binding site for the Ca(2+)-channel-blocking 1,4-dihydropyridine, (+)-[3H]PN 200-110. Radioligand binding was saturable (Bmax. = 18 +/- 2 fmol/mg of protein), reversible [Ki for (+)-PN 200-110 = 37 +/- 1 pM) and allosterically modulated by the phenylalkylamine (-)-desmethoxyverapamil [(-)-D888] as well as the bis-benzylisoquinoline alkaloid (+)-tetrandrine. Thus the 1,4-dihydropyridine binding in rMTC cells featured all characteristics of binding to the alpha 1-subunit of L-type Ca2+ channels. In contrast, in membranes of TT cells, which are known to lack Ca(2+)-sensitivity, no Ca(2+)-channel-specific (+)-[3H]PN 200-110 binding was detected. In voltage-clamp experiments, rMTC cells exhibited slowly inactivating Ca2+ currents which proved sensitive to (+)-PN 200-110, (-)-D888 and (+)-tetrandrine. These L-type Ca(2+)-channel blockers did not affect the Ca2+ currents in TT cells. The numbers of 1,4-dihydropyridine-sensitive Ca2+ channels in rMTC cells as calculated from both the binding studies and the whole-cell/single-channel recordings were 2000 and 7000/cell respectively. Thus qualitative and quantitative detection of 1,4-dihydropyridine-sensitive Ca2+ channels by radioligand-binding in Ca(2+)-sensitive rMTC cells, but not in Ca(2+)-insensitive TT cells, reflects the electrophysiological detection of functional Ca2+ channel in rMTC cells, but not in TT cells.

Delayed shortening and shrinkage of cochlear outer hair cells

Slow shortening of cochlear outer hair cells has been speculated to modify cochlear sensitivity. Tetanic electrical field stimulation of isolated outer hair cells from guinea pigs shortened the cells for 2-3 min. Electrical stimulation reduced cell length and volume (-13.5 +/- 1.5 and -37.3 +/- 3.0% of initial values, respectively, n = 16) and decreased the intracellular Cl- concentration. Cytochalasin B (100 microM) inhibited electrical stimulation-induced shortening but not volume reduction. The following chemicals or manipulations inhibited the responses: 10 microM furosemide, 0.1 mM 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS), 1 mM anthracene-9-carboxylic acid (AC9), 25 mM tetraethylammonium, 2.3 microM charybdotoxin (ChTX), 250 nM omega-conotoxin, and Ca(2+)-free medium. These findings suggest that both electrical stimulation-induced shortening and shrinkage of outer hair cells result not only from an actin-mediated contractile force, but also from Cl- efflux through furosemide-, DIDS-, and AC9-sensitive Cl- channels, and K+ efflux through ChTX-sensitive K+ channels.

Modulation of calcium current gating in frog skeletal muscle by conditioning depolarization

1. Ca2+ inward currents were measured by voltage clamping cut skeletal muscle fibres of the frog (Rana esculenta) in a double-Vaseline-gap system. 2. In order to study the basis of the previously described fast gating mode induced in the Ca2+ inward current by a conditioning depolarization we quantitatively analysed the response to differing features of the conditioning prepulse. 3. The faster activation seen during the second of two depolarizations was confined to the component of the inward current which could be blocked by 5 to 10 microM nifedipine. 4. By applying depolarizing conditioning pulses of gradually increasing length the time course of the transition to the fast gating mode could be determined. 5. Both the transition to the fast gating mode (point 4) caused by a depolarization and the slow inward current activated during the same depolarization showed similar voltage-dependent kinetics. 6. The kinetic change of the test current appeared to be equal when the same fractional activation was achieved at the end of the conditioning pulse independent of its duration or amplitude. 7. Flash photolysis of nifedipine in the interval between conditioning and test pulse showed that the predepolarization causes a rate-enhancing effect even though the slow channels were blocked by nifedipine during the conditioning pulse. 8. We conclude that the transition of the calcium channel from its slow to its fast gating mode is determined by the slow voltage-dependent reaction which limits the rate of channel opening under control conditions. This reaction is apparently not prevented by the binding of nifedipine and the block of current flow through the channel.

BAY K 8644 stimulates glucose-dependent rise of cytoplasmic Ca2+ in hyperpolarized pancreatic beta-cells

The effect of BAY K 8644 on the cytoplasmic Ca2+ concentration ([Ca2+]i) was studied in pancreatic beta-cells hyperpolarized by the K+ channel-activating agent diazoxide. After 50-60 min preexposure to 0-20 mM glucose in the presence of 400 microM diazoxide [Ca2+]i was close to the level in unstimulated beta-cells. The addition of 5 microM BAY K 8644 then triggered a rise of [Ca2+]i dependent on Ca2+ influx. The magnitude of the BAY K 8644 effect increased with the glucose concentration and was almost 10-fold higher in 20 mM than in the absence of the sugar. It is concluded that glucose can modulate Ca2+ entry through the voltage-dependent channels by a mechanism additional to depolarization. This action may help to explain why previous exposure to the sugar results in an augmented insulin response to a second challenge.

The role of NMDA receptors in long-term potentiation (LTP) and depression (LTD) in rat visual cortex

The purpose of the present study was to improve our understanding of the role of NMDA receptors in neocortical synaptic plasticity. In slices of rat visual cortex the field potential elicited in layer III in response to white matter stimulation consisted of two components with peak latencies at 5-8 ms (EPSP1) and 12-19 ms (EPSP2). EPSP2 appeared to be polysynaptic since it did not follow stimulation at 0.5 Hz. EPSP1 consisted of both kainate/AMPA and NMDA receptor activity, as revealed by bath application of DNQX and APV. EPSP2 displayed a variable sensitivity to bath-applied APV. Tetanic stimulation of the white matter in normal medium consistently induced long-term potentiation of EPSP1. In the presence of APV, LTP of EPSP1 was induced only when EPSP2 was still present, while there was no change, or LTD was induced, when EPSP2 was completely blocked by APV. In rat visual cortex, blockade of NMDA receptor participation in the postsynaptic response to tetanic stimulation reduces the probability for LTP induction but does not necessarily prevent LTP; synaptic strength may still change in either direction depending, in part, on factors affecting the magnitude of postsynaptic depolarization during tetanus.

Intracellular Mg2+ and magnesium depletion in isolated renal thick ascending limb cells

Magnesium reabsorption and regulation within the kidney occur principally within the cortical thick ascending limb (cTAL) cells of the loop of Henle. Fluorometry with the dye, mag-fura-2, was used to characterize intracellular Mg2+ concentration ([Mg2+]i) in single cTAL cells. Primary cell cultures were prepared from porcine kidneys using a double antibody technique (goat anti-human Tamm-Horsfall and rabbit anti-goat IgG antibodies). Basal [Mg2+]i was 0.52 +/- 0.02 mM, which was approximately 2% of the total cellular Mg. Cells cultured (16 h) in high magnesium media (5 mM) maintained basal [Mg2+]i, 0.48 +/- 0.02, in the normal range. However, cells cultured in nominally magnesium-free media possessed [Mg2+]i, 0.27 +/- 0.01 mM, which was associated with a significant increase in net Mg transport, (control, 0.19 +/- 0.03 and low Mg, 0.35 +/- 0.01 nmol.mg-1 protein.min-1) as assessed by 28Mg uptake. Mg(2+)-depleted cells were subsequently placed in high Mg solution (5 mM) and the Mg2+ refill rate was assessed by fluorescence. [Mg2+]i returned to normal basal levels, 0.53 +/- 0.03 mM, with a refill rate of 257 +/- 37 nM/s. Mg2+ entry was not changed by 5.0 mM Ca2+ or 2 mM Sr2+, Cd2+, Co2+, nor Ba2+ but was inhibited by Mn2+ approximately La3+ approximately Gd3+ approximately Zn2+ approximately Be2+ at 2 mM. Intracellular Ca2+ and 45Ca uptake was not altered by Mg depletion or Mg2+ refill, indicating that the entry is relatively specific to Mg2+. Mg2+ uptake was inhibited by nifedipine (117 +/- 20 nM/s), verapamil (165 +/- 34 nM/s), and diltiazem (194 +/- 19 nM/s) but enhanced by the dihydropyridine analogue, Bay K 8644 (366 +/- 71 nM/s). These antagonists and agonists were reversible with removal and [Mg2+]i subsequently returned to normal basal levels. Mg2+ entry rate was concentration and voltage dependent and maximally stimulated after 4 h in magnesium-free media. Cellular magnesium depletion results in increases in a Mg2+ refill rate which is dependent, in part, on de novo protein synthesis. These data provide evidence for novel Mg2+ entry pathways in cTAL cells which are specific for Mg2+ and highly regulated. These entry pathways are likely involved with renal Mg2+ homeostasis.

Parathyroid hormone-stimulated cadmium accumulation in Madin-Darby canine kidney cells

Although most renal cadmium transport occurs in proximal tubules indirect evidence suggests that distal tubules may also transport this heavy metal. Since the distal nephron is the site at which parathyroid hormone (PTH) regulates calcium absorption, we evaluated the effects of PTH on Cd2+ accumulation in Madin-Darby canine kidney (MDCK) cells. MDCK cells express a distal-like phenotype including PTH-sensitive adenylyl cyclase and stimulation of calcium transport. MDCK cells were grown to confluence in phenol red-free Dulbecco's modified Eagle's medium. PTH increased 109CdCl2 accumulation in a concentration-dependent manner over the range of 10(-11)-10(-9) M bPTH[1-34]. At 10(-9) M, PTH increased Cd2+ accumulation maximally by 205%. The PTH antagonist, bPTH[3-34], failed to augment 109Cd2+ accumulation. The dihydropyridine agonist, Bay k 8644, in the presence of PTH, increased 109Cd2+ uptake by 200% over vehicle-treated controls and by approximately 100% over PTH or Bay k 8644 alone. The apparent Km for Bay k 8644 activation was 1.3 microM. Bay k 8644-augmented 109Cd2+ uptake was competitively inhibited by the calcium channel antagonist nifedipine. No voltage dependence of Bay k 8644-amplified 109Cd2+ uptake could be detected. Based on these observations we conclude: (1) MDCK cells accumulate Cd2+; (2) PTH increases Cd2+ uptake into MDCK cells; and (3) Cd2+ entry in kidney epithelial cells is mediated, at least in part, by dihydropyridine-sensitive calcium channels.

Postnatal maturation of excitation-contraction coupling in rat ventricle in relation to the subcellular localization and surface density of 1,4-dihydropyridine and ryanodine receptors

To better understand excitation-contraction coupling in cardiac muscle, we investigated the main Ca2+ channels involved in that process in adult and neonatal rat ventricle. Voltage-dependent (L-type) Ca2+ channels and sarcoplasmic reticulum Ca2+ release channels were labeled by means of [3H] (+)-PN200-110 and [3H]ryanodine, respectively. The number of [3H]ryanodine binding sites (per gram tissue) increased more than that of [3H] (+)-PN200-110 binding sites over the postnatal period (2.1-fold versus 1.35-fold, respectively). After equilibration of microsomal fractions in density gradient, ryanodine receptors were characterized by a heavy distribution pattern that did not change appreciably between days 1 and 30 after birth. In neonatal tissue, 1,4-dihydropyridine receptors were found mainly in low-density subfractions, together with other sarcolemmal constituents, whereas in adult tissue, they were recovered predominantly in high-density subfractions, together with ryanodine receptors. Thus, after birth, and in parallel with the development of T tubules, there was a progressive concentration of L-type Ca2+ channels in junctional structures of high equilibrium density, where they were situated close to the Ca2+ release channels of the sarcoplasmic reticulum. In adult ventricle, L-type channels were, on an average, threefold more abundant in T tubules than in external sarcolemma. In parallel mechanical studies, we found that the inhibitory action of ryanodine on systolic contraction was much more pronounced in adult than in neonatal right ventricle, and that, conversely, neonatal tissue was more sensitive that adult tissue to inhibitors of L-type channels. We conclude that, in view of the presumed mechanism of Ca2+ release from the sarcoplasmic reticulum, that is, Ca(2+)-induced Ca2+ release, the predominant localization in adult rat ventricle of the major Ca2+ entry pathway in the vicinity of the Ca2+ release pathway is of great functional significance. Furthermore, owing to the relative stoichiometry of Ca2+ entry and Ca2+ release channels in junctional structures (about 1:9), a physical link between these channels is not likely to be involved in the modulation of Ca2+ release from the sarcoplasmic reticulum in cardiac muscle.

Two states of the L-type Ca2+ channel in PC12 cells: different sensitivity to 1,4-dihydropyridines

A omega-conotoxin-resistant component of high K(+)-induced [Ca2+]i increase in PC12 cells was further divided into two components by the difference in sensitivity to 1,4-dihydropyridines. The initial phase (up to 30s) was several times less sensitive to 1,4-dihydropyridines than the following plateau phase (several minutes). On the other hand, diltiazem blocked both phases with the same potency. Verapamil also was only a little more sensitive to the initial phase. All four 1,4-dihydropyridines tested showed 5-10 times more potent inhibition of (+)-[3H]PN200-110 binding in PC12 cells under the depolarizing (70 mM K+) condition than under the non-depolarizing (5 mM K+) condition. The biphasic blockade of the high K(+)-induced [Ca2+]i rise by 1,4-dihydropyridines appears to reflect their different affinities under depolarizing and non-depolarizing conditions.