Protein kinase C activates the non-selective cation channel in the rabbit portal vein

Cellular and molecular mechanisms regulating vascular tone. Part 1: basic mechanisms controlling cytosolic Ca2+ concentration and the Ca2+-dependent regulation of vascular tone

General anesthetics cause hemodynamic instability and alter blood flow to various organs. There is mounting evidence that most general anesthetics, at clinical concentrations, influence a wide variety of cellular and molecular mechanisms regulating the contractile state of vascular smooth muscle cells (i.e., vascular tone). In addition, in current anesthetic practice, various types of vasoactive agents are often used to control vascular reactivity and to sustain tissue blood flow in high-risk surgical patients with impaired vital organ function and/or hemodynamic instability. Understanding the physiological mechanisms involved in the regulation of vascular tone thus would be beneficial for anesthesiologists. This review, in two parts, provides an overview of current knowledge about the cellular and molecular mechanisms regulating vascular tone-i.e., targets for general anesthetics, as well as for vasoactive drugs that are used in intraoperative circulatory management. This first part of the two-part review focuses on basic mechanisms regulating cytosolic Ca2+ concentration and the Ca2+-dependent regulation of vascular tone.

Non-selective cationic channels of smooth muscle and the mammalian homologues of Drosophila TRP

Throughout the body there are smooth muscle cells controlling a myriad of tubes and reservoirs. The cells show enormous diversity and complexity compounded by a plasticity that is critical in physiology and disease. Over the past quarter of a century we have seen that smooth muscle cells contain--as part of a gamut of ion-handling mechanisms--a family of cationic channels with significant permeability to calcium, potassium and sodium. Several of these channels are sensors of calcium store depletion, G-protein-coupled receptor activation, membrane stretch, intracellular Ca2+, pH, phospholipid signals and other factors. Progress in understanding the channels has, however, been hampered by a paucity of specific pharmacological agents and difficulty in identifying the underlying genes. In this review we summarize current knowledge of these smooth muscle cationic channels and evaluate the hypothesis that the underlying genes are homologues of Drosophila TRP (transient receptor potential). Direct evidence exists for roles of TRPC1, TRPC4/5, TRPC6, TRPV2, TRPP1 and TRPP2, and more are likely to be added soon. Some of these TRP proteins respond to a multiplicity of activation signals--promiscuity of gating that could enable a variety of context-dependent functions. We would seem to be witnessing the first phase of the molecular delineation of these cationic channels, something that should prove a leap forward for strategies aimed at developing new selective pharmacological agents and understanding the activation mechanisms and functions of these channels in physiological systems.

Cellular Mechanism for Mibefradil-Induced Vasodilation of Renal Microcirculation

Although nifedipine and other conventional calcium antagonists elicit preferential vasodilation of renal afferent arterioles, we demonstrate that mibefradil and nickel, T-type calcium channel blockers, reverse the angiotensin II-induced constriction of both afferent and efferent arterioles. Since the angiotensin II-induced vasoconstriction involves inositol trisphosphate (IP3)-induced calcium release from the sarcoplasmic reticulum in the afferent arteriole, and both IP3- and protein kinase C (PKC)-mediated pathways in the efferent arteriole, we investigated the cellular mechanism for the mibefradil-induced dilation of angiotensin II-constricted renal arterioles, using the isolated perfused hydronephrotic rat kidney. Mibefradil caused a dose-dependent dilation of angiotensin II-constricted afferent and efferent arterioles, with 88 +/- 9% and 74 +/- 10% reversal observed at 1 micromol/L, respectively. The blockade of PKC by staurosporine did not alter the mibefradil-induced vasodilator responses of either arterioles (P > 0.5). In contrast, the pretreatment with thapsigargin, which predominantly blocked the IP3-mediated intracellular calcium release, prevented the afferent arteriolar constrictor response to angiotensin II, but caused a significant constriction of efferent arterioles. The subsequent addition of mibefradil had no effect on the efferent arteriolar diameter. Furthermore, the efferent arteriolar constriction induced by direct PKC activation by phorbol myristate acetate was refractory to mibefradil, but completely reversed by LOE908, a nonselective cation channel blocker. In summary, mibefradil markedly dilates the angiotensin II-induced renal arteriolar constriction; the action of mibefradil is most likely mediated by the inhibition of the IP3-mediated pathway, but the inhibitory action on the PKC pathway appears modest.

The developing relationship between receptor-operated and store-operated calcium channels in smooth muscle

Contraction of smooth muscle is initiated, and to a lesser extent maintained, by a rise in the concentration of free calcium in the cell cytoplasm ([Ca(2+)](i)). This activator calcium can originate from two intimately linked sources--the extracellular space and intracellular stores, most notably the sarcoplasmic reticulum. Smooth muscle contraction activated by excitatory neurotransmitters or hormones usually involves a combination of calcium release and calcium entry. The latter occurs through a variety of calcium permeable ion channels in the sarcolemma membrane. The best-characterized calcium entry pathway utilizes voltage-operated calcium channels (VOCCs). However, also present are several types of calcium-permeable channels which are non-voltage-gated, including the so-called receptor-operated calcium channels (ROCCs), activated by agonists acting on a range of G-protein-coupled receptors, and store-operated calcium channels (SOCCs), activated by depletion of the calcium stores within the sarcoplasmic reticulum. In this article we will review the electrophysiological, functional and pharmacological properties of ROCCs and SOCCs in smooth muscle and highlight emerging evidence that suggests that the two channel types may be closely related, being formed from proteins of the Transient Receptor Potential Channel (TRPC) family.

Muscarinic activation of transient inward current and contraction in canine colon circular smooth muscle cells

Muscarinic receptor mediated membrane currents and contractions were studied in isolated canine colon circular smooth muscle cells. Carbachol (10–5M) evoked a slow transient inward current that was superimposed by a transient outward current at holding potentials greater than –50 mV. Carbachol contracted the cells by 70 ± 2%. The effects of carbachol were blocked by atropine (10–6M), tetraethyl ammonium (20 mM), and BAPTA-AM (25 mM applied for 20 min). The inward current and contraction were not sensitive to diltiazem (10–5M), nitrendipine (3 × 10–7M), niflumic acid (10–5M), or N-phenylanthranilic acid (10–4M), but were gradually inhibited after repetitive stimulations in Ca2+free solution. Ni2+(2 mM) inhibited the inward current by 67 ± 4%. The inward current reversed at +15 mV. The outward component could be selectively inhibited by iberiotoxin (20 nM) or by intracellular Cs+. Repeated stimulation in the presence of cyclopiazonic acid (CPA, 3 µM) inhibited the carbachol-induced outward current and partially inhibited contraction. CPA did not inhibit the inward current. In conclusion, muscarinic receptor stimulation evoked a CPA-sensitive calcium release that caused contraction and a CPA-insensitive transient inward current was activated that is primarily carried by Ca2+ions and is sensitive to Ni2+.Key words: calcium, carbachol, smooth muscle, cyclopiazonic acid, sarcoplasmic reticulum.

Protein kinase C activates non-capacitative calcium entry in human platelets

1. In many non-excitable cells Ca2+ influx is mainly controlled by the filling state of the intracellular Ca2+ stores. It has been suggested that this store-mediated or capacitative Ca2+ entry is brought about by a physical and reversible coupling of the endoplasmic reticulum with the plasma membrane. Here we provide evidence for an additional, non-capacitative Ca2+ entry mechanism in human platelets. 2. Changes in cytosolic Ca2+ and Sr2+ were measured in human platelets loaded with the fluorescent indicator fura-2. 3. Depletion of the internal Ca2+ stores with thapsigargin plus a low concentration of ionomycin stimulated store-mediated cation entry, as demonstrated upon Ca2+ or Sr2+ addition. Subsequent treatment with thrombin stimulated further divalent cation entry in a concentration-dependent manner. 4. Direct activation of protein kinase C (PKC) by phorbol-12-myristate-13-acetate or 1-oleoyl-2-acetyl-sn-glycerol also stimulated divalent cation entry, without evoking the release of Ca2+ from intracellular stores. Cation entry evoked by thrombin or activators of PKC was abolished by the PKC inhibitor Ro-31-8220. 5. Unlike store-mediated Ca2+ entry, jasplakinolide, which reorganises actin filaments into a tight cortical layer adjacent to the plasma membrane, did not inhibit divalent cation influx evoked by thrombin when applied after Ca2+ store depletion, or by activators of PKC. Thrombin also activated Ca2+ entry in platelets in which the release from intracellular stores and store-mediated Ca2+ entry were blocked by xestospongin C. 6. These results indicate that the non-capacitative divalent cation entry pathway is regulated independently of store-mediated entry and does not require coupling of the endoplasmic reticulum and the plasma membrane. These results support the existence of a mechanism for receptor-evoked Ca2+ entry in human platelets that is independent of Ca2+ store depletion. This Ca2+ entry mechanism may be activated by occupation of G-protein-coupled receptors, which activate PKC, or by direct activation of PKC, thus generating non-capacitative Ca2+ entry alongside that evoked following the release of Ca2+ from the intracellular stores.

Histamine-induced calcium entry in rat cerebellar astrocytes: evidence for capacitative and non-capacitative mechanisms

We have investigated the effects of histamine on the intracellular calcium concentration ([Ca2+]i) of cultured rat cerebellar astrocytes using fura-2-based Ca2+ imaging microscopy. Most of the cells responded to the application of histamine with an increase in [Ca2+]i which was antagonized by the H1 receptor blocker mepyramine. When histamine was applied for several minutes, the majority of the cells displayed a biphasic Ca2+ response consisting of an initial transient peak and a sustained component. In contrast to the initial transient [Ca2+]i response, the sustained, receptor-activated increase in [Ca2+]i was rapidly abolished by chelation of extracellular Ca2+ or addition of Ni2+, Mn2+, Co2+ and Zn2+, but was unaffected by nifedipine, an antagonist of L-type voltage-activated Ca2+ channels. These data indicate that the sustained increase in [Ca2+]i was dependent on Ca2+ influx. When intracellular Ca2+ stores were emptied by prolonged application of histamine in Ca2+-free conditions, Ca2+ re-addition after removal of the agonist did not lead to an 'overshoot' of [Ca2+]i indicative of store-operated Ca2+ influx. However, Ca2+ stores were refilled despite the absence of any substantial change in the fura-2 signal. Depletion of intracellular Ca2+ stores using cyclopiazonic acid in Ca2+-free saline and subsequent re-addition of Ca2+ to the saline resulted in an increase in [Ca2+]i that was significantly enhanced in the presence of histamine. The results suggest that besides capacitative mechanisms, a non-capacitative, voltage-independent pathway is involved in histamine-induced Ca2+ entry into cultured rat cerebellar astrocytes.

Role of protein kinase C in angiotensin II-induced constriction of renal microvessels

Role of protein kinase C in angiotensin II-induced constriction of renal microvessels.

BACKGROUND

Although angiotensin II (Ang II) exerts its action through multiple vasomotor mechanisms, the contribution of phosphoinositol hydrolysis products to Ang II-induced renal vasoconstriction remains undetermined.

METHODS

The role of protein kinase C (PKC) in Ang II-induced afferent (AFF) and efferent (EFF) arteriolar constriction was examined using the isolated perfused hydronephrotic rat kidney.

RESULTS

Ang II (0.3 nmol/L)-induced EFF constriction was refractory to inhibition of voltage-dependent calcium channels by pranidipine (1 micromol/L, 19 +/- 2% reversal) but was completely reversed by a PKC inhibitor, chelerythrine (1 micromol/L, 96 +/- 2% reversal). Furthermore, direct PKC activation by phorbol myristate acetate (PMA; 1 micromol/L) caused prominent EFF constriction, and this constriction was inhibited by manganese and free calcium medium. In contrast, Ang II-induced AFF constriction was completely abolished by pranidipine (98 +/- 4% reversal) and was partially inhibited by chelerythrine (55 +/- 3% reversal). Although PMA elicited marked AFF constriction, this constriction was insensitive to the calcium antagonist, but was totally inhibited by manganese or free calcium medium.

CONCLUSIONS

PKC plays an obligatory role in Ang II-induced EFF constriction that requires extracellular calcium entry through nonselective cation channels. In contrast, in concert with our recent findings demonstrating a complete dilation by thapsigargin, Ang II-induced AFF constriction is mainly mediated by inositol trisphosphate (IP3) and voltage-dependent calcium channel pathways, but could not be attributed to the PKC-activated calcium entry pathway (for example, nonselective cation channels). Rather, Ang II-stimulated PKC may cross-talk to the IP3/voltage-dependent calcium channel pathway and could modulate the vasoconstrictor mechanism of the AFF. Thus, the role of PKC during Ang II stimulation differs in AFF and EFF, which may constitute segmental heterogeneity in the renal microvasculature.

Association of alcohol in brain injury, headaches, and stroke with brain-tissue and serum levels of ionized magnesium: a review of recent findings and mechanisms of action

Although there is general agreement that chronic ingestion of alcohol poses great risks for normal cardiovascular functions and peripheral-vascular homeostasis, a direct cause and effect between the real phenomena of alcohol-induced headache and risk of brain injury and stroke is not appreciated. "Binge drinking" of alcohol is associated with an ever-growing number of strokes and sudden death. It is becoming clear that alcohol ingestion can result in profoundly different actions on the cerebral circulation (e.g., vasodilation, vasoconstriction-spasm, vessel rupture), depending upon dose and physiologic state of host. Using rats, it has been demonstrated that acute, high doses of ethanol can result in stroke-like events concomitant with alterations in brain bioenergetics. We review recent in vivo findings obtained with 31P-NMR spectroscopy, optical reflectance spectroscopy, and direct in vivo microcirculatory studies on the intact brain. Alcohol-induced hemorrhagic stroke is preceded by a rapid fall in brain intracellular free magnesium ions ([Mg2+]i) followed by cerebrovasospasm and reductions in phosphocreatine (PCr)/ATP ratio, intracellular pH, and the cytosolic phosphorylation potential (CPP) with concomitant rises in deoxyhemoglobin (DH), mitochondrial reduced cytochrome oxidase aa3 (rCOaa3), blood volume, and intracellular inorganic phosphate (Pi). Using osmotic mini-pumps implanted in the third cerebral ventricle, containing 30% ethanol, it was found that brain [Mg2+]i is reduced 30% after 14 days; brain PCr fell 15%, whereas the CPP fell 40%. Such animals became susceptible to stroke from nonlethal doses of ethanol. Human subjects with mild head injury have been found to exhibit early deficits in serum ionized Mg (IMg2+); the greater the degree of early head injury (30 min-8 h), the greater and more profound the deficit in serum IMg2+ and the greater the ionized Ca (ICa2+) to IMg2+ ratio. Patients with histories of alcohol abuse or ingestion of alcohol prior to head injury exhibited greater deficits in IMg2+ (and higher ICa2+/IMg2+ ratios) and, unlike the subjects without alcohol, did not leave the hospital for at least several days. Women, for some unknown reason, exhibit a much higher incidence of morbidity and mortality from subarachnoid hemorrhage (SAH) than men. Data on 105 men and women with different types of stroke indicate that, on the average, a 20% deficit in serum IMg2+ is seen; total Mg (TMg) or blood pH is usually near normal. Women with SAH, however, exhibit much lower IMg2+ and higher ICa2+/IMg2+ ratios; the presence of ethanol in the blood is associated with even more depression in IMg2+ in SAH in women. It is possible that prior alcohol ingestion is, in large measure, responsible for a great deal of this unexplained higher incidence of SAH in women. It has recently been reported that the cyclical changes in estrogenic hormones appear to control the serum IMg2+ level in young women. A surge in estrogenic levels prior to SAH could thus precipitate, in part, the SAH. In other human studies, it has been shown that migraines and headache, dizziness, and hangover, which accompany ethanol ingestion, are associated with rapid deficits in serum IMg2+ but not in TMg. The former, and the alcohol-associated headache, can be ameliorated with IV administration of MgSO4. Premenstrual tension-headache (PTH) and its exacerbation by alcohol in women is also accompanied by deficits in IMg2+, and elevation in serum ICa2+/IMg2+; IV MgSO4 corrects the PTH and the serum deficit in IMg2+. Animal experiments show that IV Mg2+ can prevent alcohol-induced hemorrhagic stroke and the subsequent fall in brain [Mg2+]i, [PCr], pHi, and CPP. Other recent data indicate that alcohol-induced cellular loss of [Mg2+]i is associated with cellular Ca2+ overload and generation of oxygen-derived free radicals; chronic pretreatment with vitamin E prevents alcohol-induced vascular injury and pathology in the brain. (ABSTRACT TRUNCATED)

A non-capacitative pathway activated by arachidonic acid is the major Ca2+ entry mechanism in rat A7r5 smooth muscle cells stimulated with low concentrations of vasopressin

1. Depletion of the Ca2+ stores of A7r5 cells stimulated Ca2+, though not Sr2+, entry. Vasopressin (AVP) or platelet-derived growth factor (PDGF) stimulated Sr2+ entry. The cells therefore express a capacitative pathway activated by empty stores and a non-capacitative pathway stimulated by receptors; only the former is permeable to Mn2+ and only the latter to Sr2+. 2. Neither empty stores nor inositol 1,4,5-trisphosphate (InsP3) binding to its receptors are required for activation of the non-capacitative pathway, because microinjection of cells with heparin prevented PDGF-evoked Ca2+ mobilization but not Sr2+ entry. 3. Low concentrations of Gd3+ irreversibly blocked capacitative Ca2+ entry without affecting AVP-evoked Sr2+ entry. After inhibition of the capacitative pathway with Gd3+, AVP evoked a substantial increase in cytosolic [Ca2+], confirming that the non-capacitative pathway can evoke a significant increase in cytosolic [Ca2+]. 4. Arachidonic acid mimicked the effect of AVP on Sr2+ entry without stimulating Mn2+ entry; the Sr2+ entry was inhibited by 100 microM Gd3+, but not by 1 microM Gd3+ which completely inhibited capacitative Ca2+ entry. The effects of arachidonic acid did not require its metabolism. 5. AVP-evoked Sr2+ entry was unaffected by isotetrandrine, an inhibitor of G protein-coupled phospholipase A2. U73122, an inhibitor of phosphoinositidase C, inhibited AVP-evoked formation of inositol phosphates and Sr2+ entry. The effects of phorbol esters and Ro31-8220 (a protein kinase C inhibitor) established that protein kinase C did not mediate the effects of AVP on the non-capacitative pathway. An inhibitor of diacylglycerol lipase, RHC-80267, inhibited AVP-evoked Sr2+ entry without affecting capacitative Ca2+ entry or release of Ca2+ stores. 6. Selective inhibition of capacitative Ca2+ entry with Gd3+ revealed that the non-capacitative pathway is the major route for the Ca2+ entry evoked by low AVP concentrations. 7. We conclude that in A7r5 cells, the Ca2+ entry evoked by low concentrations of AVP is mediated largely by a non-capacitative pathway directly regulated by arachidonic acid produced by the sequential activities of phosphoinositidase C and diacylglycerol lipase.

M2 signaling in smooth muscle cells

M2 receptor stimulation results in the gating of nonselective cation channels in several smooth muscle cell types. However the requirement for current activation includes a rise in cytosolic calcium mediated by M3 receptor induced calcium release. This complex signaling system confers substantial complexity on the interpretation of pharmacological experiments. M2 and M3 receptor stimulation has also been linked to the inhibition of potassium channels in smooth muscle. These signaling events are likely to play important roles in excitation/contraction coupling.

Physiological features of visceral smooth muscle cells, with special reference to receptors and ion channels

Visceral smooth muscle cells (VSMC) play an essential role, through changes in their contraction-relaxation cycle, in the maintenance of homeostasis in biological systems. The features of these cells differ markedly by tissue and by species; moreover, there are often regional differences within a given tissue. The biophysical features used to investigate ion channels in VSMC have progressed from the original extracellular recording methods (large electrode, single or double sucrose gap methods), to the intracellular (microelectrode) recording method, and then to methods for recording from membrane fractions (patch-clamp, including cell-attached patch-clamp, methods). Remarkable advances are now being made thanks to the application of these more modern biophysical procedures and to the development of techniques in molecular biology. Even so, we still have much to learn about the physiological features of these channels and about their contribution to the activity of both cell and tissue. In this review, we take a detailed look at ion channels in VSMC and at receptor-operated ion channels in particular; we look at their interaction with the contraction-relaxation cycle in individual VSMC and especially at the way in which their activity is related to Ca2+ movements and Ca2+ homeostasis in the cell. In sections II and III, we discuss research findings mainly derived from the use of the microelectrode, although we also introduce work done using the patch-clamp procedure. These sections cover work on the electrical activity of VSMC membranes (sect. II) and on neuromuscular transmission (sect. III). In sections IV and V, we discuss work done, using the patch-clamp procedure, on individual ion channels (Na+, Ca2+, K+, and Cl-; sect. IV) and on various types of receptor-operated ion channels (with or without coupled GTP-binding proteins and voltage dependent and independent; sect. V). In sect. VI, we look at work done on the role of Ca2+ in VSMC using the patch-clamp procedure, biochemical procedures, measurements of Ca2+ transients, and Ca2+ sensitivity of contractile proteins of VSMC. We discuss the way in which Ca2+ mobilization occurs after membrane activation (Ca2+ influx and efflux through the surface membrane, Ca2+ release from and uptake into the sarcoplasmic reticulum, and dynamic changes in Ca2+ within the cytosol). In this article, we make only limited reference to vascular smooth muscle research, since we reviewed the features of ion channels in vascular tissues only recently.

Neurotransmitter-induced novel modulation of a nonselective cation channel by a cAMP-dependent mechanism in rat pineal cells

In the rat, circadian rhythm in melatonin is regulated by noradrenergic and neuropeptide inputs to the pineal via adenosine 3',5'-cyclic monophosphate (cAMP)- and Ca2+-dependent mechanisms. We have identified a large conductance (170 pS), voltage-dependent, nonselective cation channel on rat pineal cells in culture that shows a novel mode of modulation by cAMP. Pituitary adenylate cyclase activating peptide (PACAP), norepinephrine, or 8-Br-cAMP increase channel open probability (Po) with a hyperpolarizing shift in voltage dependence such that the channel becomes active at resting membrane potentials. The increase in Po was accompanied by a change in current rectification properties such that the channel was transformed from being inactive at rest to an inwardly rectifying cation conductance in the presence of agonist, which depolarizes the cell. This channel is calcium insensitive, is blocked by Cs+, and shows a permeability sequence: K+ > Na+ >/= NH+4 > Li+. The data suggest that PACAP and norepinephrine acting through a cAMP-dependent mechanism modulate this nonselective cation channel, resulting in a slow onset depolarization that may be important in regulation of pineal cell excitability.

Alpha-tocopherol prevents ethanol-induced elevation of [Ca2+]i in cultured canine cerebral vascular smooth muscle cells

Exposure of cultured canine cerebral vascular smooth muscle cells to ethanol (10-400 mM) for 1-5 days results in concentration-dependent elevation in resting intracellular free calcium ([Ca2+]i) levels. Preincubation of these cultured vascular cells with alpha-tocopherol (20 microM), alone, did not produce any apparent changes from control resting levels of [Ca2+]i. However, after concomitant addition of alpha-tocopherol (20 microM) and ethanol (10-400 mM), the rises of [Ca2+]i induced by ethanol were attenuated markedly. These results suggest that alcohol-induced lipid peroxidation of cerebral vascular muscle cell membranes triggers membrane entry of extracellular Ca2+, which could play an important role in ethanol-induced cerebrovasospasm, brain ischemia and stroke. Moreover, these new results support the concept recently advanced to suggest that alpha-tocopherol-induced amelioration of membrane lipid alterations of cerebral vascular cells can prevent ethanol-induced excessive accumulation of [Ca2+]i.

Cellular mechanisms mediating rat renal microvascular constriction by angiotensin II

To assess cellular mechanisms mediating afferent (AA) and efferent arteriolar (EA) constriction by angiotensin II (AngII), experiments were performed using isolated perfused hydronephrotic kidneys. In the first series of studies, AngII (0.3 nM) constricted AAs and EAs by 29+/-3 (n = 8, P < 0.01) and 27+/-3% (n = 8, P < 0.01), respectively. Subsequent addition of nifedipine restored AA but not EA diameter. Manganese (8 mM) reversed EA constriction by 65+/-9% (P < 0.01). In the second group, the addition of N-ethylmaleimide (10 microM), a Gi/Go protein antagonist, abolished AngII- induced EA (n = 6) but not AA constriction (n = 6). In the third series of experiments, treatment with 2-nitro-4-carboxyphenyl-N, N-diphenyl-carbamate (200 microM), a phospholipase C inhibitor, blocked both AA and EA constriction by AngII (n = 6 for each). In the fourth group, thapsigargin (1 microM) prevented AngII-induced AA constriction (n = 8) and attenuated EA constriction (8+/-2% decrease in EA diameter at 0.3 nM AngII, n = 8, P < 0.05). Subsequent addition of manganese (8 mM) reversed EA constriction. Our data provide evidence that in AAs, AngII stimulates phospholipase C with subsequent calcium mobilization that is required to activate voltage-dependent calcium channels. Our results suggest that AngII constricts EAs by activating phospholipase C via the Gi protein family, thereby eliciting both calcium mobilization and calcium entry.

M2 receptor activation of nonselective cation channels in smooth muscle cells: calcium and Gi/G(o) requirements

Muscarinic stimulation of fura 2-loaded smooth muscle cells evoked a rapidly inactivating Ca(2+)-activated Cl- current [ICl(Ca)] and a sustained nonselective cation current (Icat) as well as a transient (delta Ca(tran)) and a sustained (delta Ca(sus)) elevation of cytosolic Ca2+ concentration ([Ca2+]i). Caffeine and inositol 1,4,5-trisphosphate induced delta Ca(tran) and ICl(Ca) but not Icat or delta Ca(sus). M2 receptor antagonism blocked muscarinic activation of Icat and delta Ca(sus) but not ICl(Ca) and delta Ca(tran). M3 antagonism blocked activation of ICl(Ca) and Icat and a rise in [Ca2+]i, but application of caffeine with methacholine restored Icat and delta Ca(sus). After depletion of intracellular Ca2+ stores, methacholine failed to induce Icat or a [Ca2+]i increase and, in pertussis toxin-treated cells, ICl(Ca) and delta Ca(tran) but not Icat or delta Ca(sus) were evoked. Anti-G alpha i-1/G alpha i-2 antibodies and anti-G alpha i-3/ G(o) alpha antibodies blocked Icat but did not affect ICl(Ca). Anti-Gq alpha/ G alpha 11 antibodies greatly inhibited ICl(Ca) but did not affect Icat. Activation of M2 receptors leads to the opening of nonselective cation channels through Gi/G(o) proteins in smooth muscle cells, resulting in a sustained rise in [Ca2+]i. Arise in [Ca2+]i is necessary but not sufficient for activation of nonselective cation channels.

Ca(2+)-activated Cl- currents are activated by metabolic inhibition in rat pulmonary artery smooth muscle cells

We report the electrophysiological and functional properties of Ca(2+)-activated Cl- currents [ICl(Ca)] in rat pulmonary artery smooth muscle and the activation of these currents by the metabolic inhibitor cyanide. Caffeine and norepinephrine (NE) evoked both Ca(2+)-activated K+ currents [IK(Ca)] and ICl(Ca) currents in voltage-clamped myocytes (-50 mV). Niflumic acid (10 microM) reduced the caffeine-induced ICl(Ca) by approximately 64% and reversibly reduced NE-induced tension. Exposure of myocytes to cyanide (2-10 mM) induced a slowly developing inward current (-50 mV) in physiological and K(+)-free solutions, which was identified as ICl(Ca) on the basis of ion selectivity and Ca2+ dependence. Cyanide elevated cytosolic Ca2+ concentration, and this elevation was markedly inhibited by preexposure to caffeine and slightly inhibited by nisoldipine. During exposure to caffeine, the Ca(2+)-activated K+ current was also augmented. Cyanide markedly prolonged ICl(Ca) activated by caffeine, increasing the half-decay time from 3.5 (control) to 29 s (cyanide); the half-decay time of the caffeine-induced IK(Ca) was not significantly affected by cyanide. The results indicate that metabolic inhibition increases [Ca2+]i and activates a prolonged, depolarizing Cl- current in pulmonary artery myocytes.

Alpha 1-adrenoceptor activation of a non-selective cation current in rabbit portal vein by 1,2-diacyl-sn-glycerol

1. The transduction mechanisms involved in the activation and modulation of the noradrenaline-activated cation current (Icat) were investigated with whole-cell patch clamp techniques in rabbit portal vein smooth muscle cells. 2. Intracellular application of guanosine 5-O-(3-thiotriphosphate) (GTP gamma S, 500 microM) evoked a 'noisy' inward current at -50 mV with a similar current-voltage relationship and reversal potential to the current evoked by bath application of noradrenaline (100 microM). Guanosine 5-O-(2-thiodiphosphate) (GDP beta S, 1 mM) markedly inhibited noradrenaline-activated Icat. 3. The phospholipase C (PLC) inhibitor U73122 inhibited the amplitude of the noradrenaline-activated Icat in a concentration- and time-dependent manner and the IC50 was about 180 nM. U73122 had similar effects on the cation current evoked by GTP gamma S. 4. Intracellular application of myo-inositol 1,4,5-trisphosphate (IP3, 100 microM) from the patch pipette did not activate any membrane current in cells where intracellular calcium concentration ([Ca2+]i) was buffered to 14 nM, but subsequent addition of noradrenaline evoked Icat. 5. Bath application of the 1,2-diacyl-sn-glycerol (DAG) analogue 1-oleoyl-2-acetyl-sn-glycerol (OAG, 10 microM) activated Icat, whereas the phorbol ester phorbol 12,13-dibutyrate (PDBu, 0.1-5 microM) failed to activate Icat, in every cell examined. Icat activated by OAG after bath application of PDBu was not significantly different from OAG-activated Icat in the absence of PDBu. The DAG lipase inhibitor RHC80267 (10 microM) activated Icat in some cells, whereas the DAG kinase inhibitor R59949 (10 microM) never activated Icat. 6. Bath application of the protein kinase C inhibitor chelerythrine (1-10 microM) had no effect on either OAG-or noradrenaline-activated Icat. 7. It is concluded that noradrenaline activates Icat via a G-protein coupled to PLC and that the resulting DAG product plays a central role in the activation of cation channels via a protein kinase C-independent mechanism.

Shrinkage activates a nonselective conductance: involvement of a Walker-motif protein and PKC

The ability of all cells to maintain their volume during an osmotic challenge is dependent on the regulated movement of salt and water across the plasma membrane. We demonstrate the phosphorylation-dependent gating of a nonselective conductance in Caco-2 cells during cellular shrinkage. Intracellular application of exogenous purified rat brain protein kinase C (PKC) resulted in the activation of a current similar to that activated during shrinkage with a Na(+)-to-Cl- permeability ratio of approximately 1.7:1. To prevent possible PKC- and/or shrinkage-dependent activation of cystic fibrosis transmembrane regulator (CFTR), which is expressed at high levels in Caco-2 cells, a functional anti-peptide antibody, anti-CFTR505-511, was introduced into the cells via the patch pipette. Anti-CFTR505-511, which is directed against the Walker motif in the first nucleotide binding fold of CFTR, prevented the PKC/shrink-age current activation. The peptide CFTR505-511 also induced current inhibition, suggesting the possible involvement of a regulatory element in close proximity to the channel that shares sequence homology with the first nucleotide binding fold of CFTR and whose binding to the channel is required for channel gating.

Action of angiotensin II, 5-hydroxytryptamine and adenosine triphosphate on ionic currents in single ear artery cells of the rabbit

1. Angiotensin II, 5-hydroxytryptamine (5-HT) and adenosine triphosphate (ATP) evoked a transient inward current in isolated single car artery cells of rabbit held at -60 mV by whole cell voltage clamp in physiological saline using a KCL-containing pipette solution. Under these conditions agonist did not activate a calcium-dependent potassium current. 2. Responses to each agonist were transient and desensitized rapidly. Inward current at -60 mV holding potential was not abolished by blockade of voltage-dependent calcium channels or by buffering intracellular calcium with BAPTA, a calcium chelator, or following depletion of intracellular calcium stores with ryanodine. 3. The shape of the current-voltage relationships and the reversal potentials of the current induced by angiotensin II, 5-HT and ATP were similar under a variety of ionic conditions. Agonist-induced current was unaffected by replacing intracellular chloride with citrate ions or by replacing intracellular sodium with caesium or extracellular sodium with barium or calcium. Replacement of extracellular sodium with Tris shifted the reversal potential in all cases by around 30 mV negatively. 4. These data suggest that angiotensin II, 5-HT and ATP activate similar cationic conductances which are relatively non-selective allowing mono- and divalent cations to cross the smooth muscle cell membrane. These channels may allow the influx of calcium under physiological conditions.

Vasopressin stimulation of Ca2+ mobilization, two bivalent cation entry pathways and Ca2+ efflux in A7r5 rat smooth muscle cells

1. Arg8-vasopressin (AVP)-regulated Ca2+ transport were investigated in fura-2-loaded A7r5 cells using both single cell and population measurements. 2. AVP evokes an initial concentration-dependent rise in cytosolic free Ca2+ concentration ([Ca2+ ]i) to a peak which is independent of extracellular Ca2+, and a sustained Ca2+ signal that results from a balance between stimulation of Ca2+ entry and efflux. 3. Depletion of intracellular Ca2+ stores with thapsigargin, ionomycin, or prior treatment with AVP in Ca2(+)-free medium activates 'capacitative' entry of Ca2+, Ba2+ or Mn2+. Capacitative Mn2+ entry is inhibited by refilling stores with Ca2+; neither Sr2+ nor Ba2+ substitute for Ca2+ to give this effect. 4. In cells with empty stores, AVP stimulates further bivalent cation entry, and the effect persists when extracellular Na+ is replaced by N-methyl-D-glucamine or under depolarizing condition (extracellular KCl concentration ([KCl]o), 135 mM). This effect of AVP is not therefore merely a consequence of AVP causing membrane hyperpolarization or stimulation of Na(+)-Ca2+ exchange, but results from opening of a bivalent cation influx pathway. 5. Several lines of evidence indicate that AVP-stimulated bivalent cation entry is not a consequence of more complete emptying of the intracellular stores and consequent further activation of the capacitative pathway. AVP stimulates Ba2+ entry when the intracellular Ca2+ stores have been both emptied by ionomycin and prevented from refilling by thapsigargin. Mn2+ permeates the capacitative pathway, but AVP does not further increase Mn2+ entry, confirming that AVP does not further activate the capacitative pathway and that the two pathways differ in their permeability to Mn2+. When the extracellular [Sr2+] is low, empty stores do not stimulate detectable Sr2+ entry, but addition of AVP causes substantial Sr2+ entry. 6. A decrease in [Ca2+]i occurs when 50 nM AVP is added during a sustained elevation of [Ca2+]i evoked by thapsigargin. Since AVP does not inhibit the capacitative pathway, this result suggests that AVP stimulates Ca2+ extrusion. 7. We conclude that stimulation of Ca2+ mobilization, two modes of bivalent cation entry, and Ca2+ efflux all contribute to the complex concentration-dependent effects of AVP in A7r5 smooth muscle cells.

Non-selective cation channels in cerebrovascular smooth muscle cells of adult rats

Inside-out patch clamp recordings have revealed the presence of a novel, large conductance channel of the non-selective, cation permeable type in smooth muscle cells dispersed from the cerebral arteries of adult rats. In physiologically appropriate ionic gradients, current flow in these channels reversed polarity at a membrane potential of about -42 mV. Single channel conductance in symmetrical 140 mM K+ salines was 211 pS. The channel was permeable to both K+ and Na+, with a ratio PNa/PK = 0.15, while Cl- was effectively impermeant. Calcium ions were weakly permeant (PCa/PK = 0.03, PCa/PNa = 0.20). Channel open probability increased with membrane depolarization and was weakly dependent on the concentration of free intracellular Ca2+. This channel would contribute outward membrane current at potentials more positive than about -42 mV. In concert with outward potassium currents, it may serve to limit membrane depolarization during action potential activity in cerebrovascular smooth muscle cells.

Simultaneous detection of free radical release and membrane current during phagocytosis

Stimulation of macrophages induces the "respiratory burst" response which is associated with the generation of superoxide (O2-), a drop in cytoplasmic pH, and a pronounced depolarization of the membrane potential. The purpose of the present studies was to determine whether an increase in O2- was temporally related to changes in membrane potential and transmembrane current. Release of O2- at the single cell level was photometrically monitored during phagocytosis of immune complexes while simultaneously measuring whole-cell current. Membrane depolarization and the generation of a non-selective current followed an increase in O2- production with a variable lag time which was correlated with the state of cellular maturation in culture. In the absence of phagocytosis, the exposure of macrophages to O2- generated by a xanthine-xanthine oxidase reaction activated a non-selective current similar to that seen after phagocytosis. These results provide the first demonstration of the relationship between free radical release and the ensuing electrophysiological signaling events which are linked to particle engulfment in phagocytic cells.

Ca2+ handling mechanisms underlying neuropeptide Y-induced contraction in canine basilar artery

The effects of neuropeptide Y on isometric tension simultaneously measured with cytosolic Ca2+ concentration ([Ca2+]cyt) and Ca2+ sensitivity of contractile elements were studied in isolated canine basilar arteries. Neuropeptide Y (1-100 nM) increased [Ca2+]cyt and tension in a concentration-dependent and parallel manner, whereas 9,11-dideoxy-11 alpha,9 alpha-epoxymethano prostaglandin F2 alpha (U46619) (10-100 nM), a thromboxane A2 mimetic, produced a large contraction with a small increase in [Ca2+]cyt. Ca2+ channel antagonists such as d-cis-diltiazem (10 mM) abolished both [Ca2+]cyt and tension augmented by neuropeptide Y. In Ca(2+)-free solution containing 0.2 mM EGTA, neuropeptide Y did not change [Ca2+]cyt and tension, whereas U46619 transiently increased both of them. Furthermore, neuropeptide Y apparently did not affect the Ca2+ sensitivity when assessed in the artery permeabilized with Staphylococcus aureus alpha-toxin, whereas U46619 augmented it. These findings suggest that neuropeptide Y-induced contraction in the canine basilar artery is produced mainly by Ca2+ influx through L-type Ca2+ channels.