Modulation of Ca current during the phosphorylation cycle in the guinea pig heart

Potentiation by cyclic GMP of beta-adrenergic effect on Ca2+ current in guinea-pig ventricular cells

1. Effects of cyclic GMP on L-type Ca2+ current (ICa) were investigated in myocytes isolated from guinea-pig ventricles using the patch clamp method in the whole-cell configuration combined with intracellular perfusion. 2. When ICa was increased by bath application of isoprenaline (0.001-0.1 microM) or forskolin (0.5-1 microM), or by intracellular dialysis with cyclic AMP (50-100 microM), dialysis with 10 microM-cyclic GMP resulted in an additional stimulation of ICa. Without these pre-treatments, cyclic GMP (1-100 microM) had no effect on the basal ICa. 5'-GMP was without effect. 3. The stimulatory effect of cyclic GMP was observed at concentrations higher than 0.1 microM with a maximum at around 10 microM in the pipette. The dose-response relation between isoprenaline and ICa was shifted to the left by (10 microM) cyclic GMP; the half-maximum isoprenaline concentration shifted from 16 to 4.6 nM. 4. The increase of ICa on dialysing 50 microM-cyclic AMP varied from cell to cell, probably due to a difference in phosphodiesterase activity. The cells responding weakly to cyclic AMP showed a greater response to cyclic GMP, and vice versa. In cells dialysed with hydrolysis-resistant derivatives (10-50 microM-8-(4-chlorophenylthio)-cyclic AMP or 50 microM-8-bromo-cyclic AMP), additional dialysis with cyclic GMP failed to modify ICa. Dialysis with cyclic GMP abolished the stimulatory effect of milrinone, a specific inhibitor of cyclic GMP-inhibited phosphodiesterase. These findings suggested that inhibition of cyclic GMP-sensitive phosphodiesterase was responsible for the stimulatory effect of cyclic GMP. 5. In the presence of isoprenaline, direct application of an active fragment of cyclic GMP-dependent protein kinase (PKG) failed to modify ICa in most cells. Activation of native PKG by intracellular dialysis with 8-bromo-cyclic GMP, or higher concentrations of cyclic GMP (100-1000 microM), depressed ICa in about 25% of the cells. Furthermore, dialysis of cyclic GMP reversed the increase of ICa by the non-specific phosphodiesterase inhibitor, 3-isobutyl-1-methyl-xanthine (IBMX). These findings suggested the presence of antagonistic mechanisms of cyclic GMP, which are independent from the above synergistic action. PKG may be involved in this antagonistic effect.

Enzymatic gating of voltage-activated calcium channels

The model of calcium-channel gating described above, although almost certainly too simple, suggests a direct role for protein kinases and phosphatases in determining the kinetics of calcium channel gating on a subsecond time scale. In addition, it provides a unique perspective for understanding studies of calcium channel gating under widely different metabolic and pharmacological conditions. Although many of these effects may be specific to the dihydropyridine-sensitive or L-type calcium channel, they give an indication of the range of possibilities for integrating calcium-channel activity with cellular biochemistry.

Antisense oligodeoxynucleotide to the cystic fibrosis gene inhibits anion transport in normal cultured sweat duct cells

We have tested the hypothesis that the cystic fibrosis (CF) gene product, called the CF transmembrane conductance regulator (CFTR), mediates anion transport in normal human sweat duct cells. Sweat duct cells in primary culture were treated with oligodeoxynucleotides that were antisense to the CFTR gene transcript in order to block the expression of the wild-type CFTR. Anion transport in CFTR transcript antisense-treated cells was then assessed with a halide-specific dye, 6-methoxy-N-(3-sulfopropyl)quinolinium, and fluorescent digital imaging microscopy to monitor halide influx and efflux from single sweat duct cells. Antisense oligodeoxynucleotide treatment (3.9 or 1.3 microM) for 24 hr virtually abolished Cl- transport in sweat duct cells compared with untreated cells or control cells treated with sense oligodeoxynucleotides. Br- uptake into sweat duct cells was also blocked after a 24-hr CFTR transcript antisense treatment, but not after treatment for only 4 hr. Lower concentrations of antisense oligodeoxynucleotides were less effective at inhibiting Cl- transport. These results indicate that oligodeoxynucleotides that are antisense to CFTR transcript inhibit sweat duct Cl- permeability in both a time-dependent and dose-dependent manner. This approach provides evidence that inhibition of the expression of the wild-type CFTR gene in a normal, untransfected epithelial cell results in an inhibition of Cl- permeability.

Influence of beta-adrenergic stimulation on the fast sodium current in the intact rat papillary muscle

The loose-patch-clamp technique was used on intact cardiac papillary muscle of the rat to examine whether the fast sodium inward current (INa+) is influenced by the beta-adrenergic stimulant isoproterenol (ISO) or by 8-bromo-3',5'-cyclic adenosine monophosphate (8-Br-cAMP), respectively. The amplitude of INa+ evoked by test pulses of 5 ms to a transmembrane potential of 0 mV and its time to peak were analyzed. The availability of INa+ was tested with conditioning pulses of 2.5 s to potentials between -130 mV and -50 mV. The potential of half-maximal availability was slightly shifted to more negative values by 1 microM ISO (2.0 mV, n.s.), as well as by 50 microM 8-Br-cAMP (4.0 mV; p less than 0.05). The peak amplitude of INa+ elicited from strongly negative potentials was increased by ISO (18%, n.s.), while 8-Br-cAMP exerted no directional effect. Depolarizing conditioning pulses (-60 mV) decreased INa+ to 13.3% of the maximal attainable current under control conditions, while ISO decreased INa+ to 9.1% of control (p less than 0.1). Corresponding values under the influence of 8-Br-cAMP were 11.4% and 8.3% (p less than 0.05). Moreover, in the presence of ISO there was a significant shortening of the time to peak of INa+ (0.56 ms to 0.50 ms at -80 mV conditioning potential, p less than 0.05) which could not be detected in the presence of 8-Br-cAMP.(ABSTRACT TRUNCATED AT 250 WORDS)

Intracellular Na+ modulates the cAMP-dependent regulation of ion channels in the heart

The cAMP-dependent regulation of ion channels was studied by using the whole-cell configuration of the patch clamp technique. In isolated cardiac ventricular myocytes, the beta-adrenergically regulated Cl- current (ICl) exhibited an unusual dependence on Na+, such that replacement of extracellular Na+ with compounds such as tetramethylammonium, choline, Tris, or N-methyl-D-glucamine resulted in a reduction in current amplitude without changing the reversal potential. Replacement of extracellular Na+ with tetramethylammonium also reduced the magnitude of the beta-adrenergically enhanced Ca2+ current and delayed rectifier K+ current, suggesting that removal of Na+ was affecting the cAMP pathway that regulates all three currents. Replacement of extracellular Na+ also reduced ICl that was stimulated by (i) direct activation of adenylate cyclase with forskolin, (ii) inhibition of phosphodiesterase with 3-isobutyl-1-methylxanthine, (iii) exposure to the membrane-permeable cAMP derivative 8-bromoadenosine 3',5'-cyclic monophosphate, or (iv) direct phosphorylation of the channel with protein kinase A catalytic subunit. This suggests that the Na+ dependence is at a point beyond the activation of protein kinase A. The Na+ dependence of ICl regulation could not be explained by changes in intracellular Ca2+. However, the sensitivity of the ICl to changes in extracellular Na+ depended significantly on the intracellular Na+ concentration, suggesting that intracellular Na+ plays an important role in the cAMP-dependent regulation of ion channels.

Recording of voltage and Ca(2+)-dependent currents in Xenopus oocytes using an intracellular perfusion method

We describe a method for internal perfusion of Xenopus laevis oocytes that allows control of the composition of intracellular and extracellular solutions, including the possibility of sequential introduction of different substances inside and outside the cell. Using this method, it was possible to record Ca2+ dependent Cl- current and to inhibit it by intracellular perfusion of EGTA-containing solution. With a high BA2+ solution at the external surface of the perfused oocyte, Ba2+ currents through voltage-dependent Ca2+ channels were observed in native and in cardiac RNA-injected oocytes. Finally, a delayed rectifier K+ current was recorded and blocked by internally perfused Cs+ in oocytes injected with mRNA of a cloned (MBK1) K+ channel. The method is expected to be useful for the study of function and modulation of ion channels and transporters in the oocyte, which is an important and widely used model system.

Sympathetic regulation of cardiac calcium current is due exclusively to cAMP-dependent phosphorylation

The positive inotropic effect of the sympathetic nervous system on the heart is partly mediated by an increase in the voltage-gated Ca2+ current (ICa). This increase is generally attributed to beta-adrenergic receptor-stimulated cyclic AMP-dependent phosphorylation of the Ca2+ channel. It has been suggested that cAMP-dependent phosphorylation cannot explain all the effects of beta-adrenergic agonists on ICa and that a parallel membrane-delimited pathway involving the 'direct' action of the G protein Gs also stimulates ICa. A precedent exists for such a membrane-delimited pathway in the activation of a K+ channel by acetylcholine in heart. A membrane-delimited pathway for stimulation of ICa might be important in rapid beat-to-beat regulation of contraction by the sympathetic nervous system, because isoproterenol may produce a biphasic increase in ICa with the rapid phase (tau = 150 ms) putatively mediated by the direct pathway and the slow phase (tau = 35 s) by cAMP-dependent phosphorylation. Here we report that in frog, rat, and guinea pig ventricular myocytes ICa increases slowly and monophasically in response to isoproterenol. The increase is completely blocked by inhibitors of cAMP-dependent phosphorylation. Furthermore, the time course of the increase in ICa closely parallels the increase in contractile force produced by sympathetic nerve stimulation. These data refute earlier suggestions that regulation of Ca2+ channels by the sympathetic nervous system involves or requires a direct G-protein pathway.

Different G proteins mediate the opioid inhibition or enhancement of evoked [5-methionine]enkephalin release

This laboratory has previously demonstrated that there is an opiate receptor-mediated, concentration-dependent modulation of the electrically stimulated release of enkephalin from the guinea pig myenteric plexus. Low doses of opioids (nanomolar) enhance release, whereas higher concentrations (10-100 nM) inhibit release. We now demonstrate that the in vivo i.p. administration of the islet-activating protein from pertussis toxin (PTX; 50 micrograms/500 g of body weight) markedly diminishes the potency of mu, delta, or kappa-selective opioids to inhibit the evoked release of enkephalin. In contrast, PTX is without effect on the enhancement of enkephalin release observed after treatment with nanomolar concentrations of the above opioids. Conversely, pretreatment with cholera toxin (CTX; 0.01 nM for 3 hr in vitro) has no effect on the mu, delta, or kappa opioid inhibition of evoked enkephalin release but abolishes the ability of nanomolar concentrations of these agonists to enhance stimulated enkephalin release. These data indicate that different classes of guanine nucleotide-binding proteins (G proteins) appear to mediate the opioid enhancement or inhibition of stimulated enkephalin release. Furthermore, they suggest that a PTX-sensitive G protein (Gi or Go) and a CTX-sensitive G protein (Gs) are integral components of the mechanism that mediates opioid inhibition and opioid enhancement, respectively, of evoked enkephalin release. To our knowledge, this report represents the first demonstration that Gs-coupled opiate receptors (in addition to those that are coupled to Gi) can modulate transmitter release.

Some aspects of heart beta adrenoceptor function

Heart rate and force can be increased by noradrenaline and adrenaline through an interaction with both beta 1-adrenoceptors (beta 1AR) and beta 2-adrenoceptors (beta 2 AR). Several ionic currents (I) can flow upon beta AR activation: ICa (through either beta 1AR or beta 2AR), INa, IK, and ICl. Calcium currents (ICa) can be increased directly by the alpha s unit of a GTP binding protein, Gs, or by coupling of Gs to adenylyl cyclase with subsequent formation of cyclic AMP, release of the catalytic unit of cyclic AMP-dependent protein kinase, and phosphorylation of calcium channels and other proteins. Chronic exposure (days or months), but not acute exposure (hours), to a catecholamine downregulates human heart beta 1AR. Acute desensitization partially uncouples human heart beta AR from the adenylyl cyclase. Both acute and chronic desensitization reduce positive inotropic responses to catecholamines. In human heart, catecholamine-induced activation of one beta 2AR causes the production of at least four times more cyclic AMP than activation of one beta 1AR. Chronic treatment of patients with beta 1AR-selective blockers paradoxically induces selective inotropic beta 2AR hyperresponsiveness, presumably by increasing coupling of beta 2AR to Gs. Several partial agonists with high affinity for heart beta 1AR and beta 2AR cause stimulant effects that are resistant to blockade of beta 1AR and beta 2AR. Such nonconventional partial agonists could perhaps interact with beta AR that resemble beta 3 adrenoceptors.

G protein-mediated ion channel activation

Guanine nucleotide binding proteins couple a wide variety of receptors to ion channels via both "direct" or membrane-delimited and "indirect" second messenger-mediated pathways. This tutorial summarizes current approaches to defining the mechanisms of guanine nucleotide binding protein-mediated ion channel activation. Two well-characterized ion channels in the heart, namely, the beta-adrenergic receptor-activated calcium channel and the muscarinic receptor-activated potassium channel, are used to illustrate the criteria that can distinguish between direct and indirect guanine nucleotide binding protein-transduced pathways.

Calpastatin and nucleotides stabilize cardiac calcium channel activity in excised patches

The activity of single L-type Ca2+ channels is rapidly lost (run-down) when contact between the membrane and cytosol is interrupted. We have now achieved the stabilization of cardiac Ca2+ channel activity of guinea-pig ventricular myocytes by using either cytosol or defined components added to excised patches. The endogenous protease inhibitor, calpastatin, together with nucleotides, ATP + GTP, was found to prevent run-down as effectively as cardiac cytosolic solution. These results suggest the involvement of proteolysis by calpain in run-down of channel activity and enable the study of cardiac Ca2+ channel regulation with free access to both sides of the membrane.

Regulation of endothelin-mediated calcium mobilization in vascular smooth muscle cells by isoproterenol

We have investigated the effect of isoproterenol on endothelin-induced Ca2+ mobilization in A10 vascular smooth muscle cells. Endothelin (ET) stimulates a rapid and sustained elevation of intracellular Ca2+ mediated by production of inositol phosphates, release of intracellular Ca2+, and activation of a plasmalemmal Ca2+ influx pathway. This influx pathway appears to be a L-type channel because it is inhibited by nicardipine and activated by BAY K 8644. Depolarization of the cells, by elevating extracellular K+, activated a pharmacologically similar channel and produced a similar change in intracellular Ca2+ concentration. Preincubation of cells with isoproterenol reduced the peak Ca2+ response to endothelin and blocked the sustained elevation. However, isoproterenol did not alter K(+)-induced Ca2+ entry. Thus it appears that ET-induced entry is mediated by intracellular signals and not by depolarization. With the use of cells incubated in Ca2(+)-free medium containing 1 mM ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid, isoproterenol was shown to inhibit Ca2+ release from intracellular pools by 36 +/- 3%. Furthermore, isoproterenol pretreatment or addition of adenosine 3',5'-cyclic monophosphate (cAMP) to saponin-permeabilized cells inhibited inositol 1,4,5-trisphosphate [Ins(1,4,5)P3]-induced Ca2+ release from intracellular sites. Similar effects were seen with forskolin. Propranolol reversed the inhibitory effects of isoproterenol. Isoproterenol pretreatment also inhibited the rapid formation of Ins(1,4,5)P3 and [2-3H]inositol 1,3,4,5-tetrakisphosphate stimulated by endothelin and reduced the sustained formation of these compounds. Finally, isoproterenol and forskolin led to a greater than 10-fold increase in intracellular cAMP levels. This stimulation of adenylate cyclase by isoproterenol was completely blocked by propranolol. It appears then that the beta-agonist isoproterenol interacts with a beta-adrenergic receptor, elevates cAMP, and thereby alters endothelin-induced Ca2+ mobilization. Inhibition of Ins(1,4,5)P3 formation, reduction in the responsiveness of the Ins(1,4,5)P3 intracellular receptor, and perhaps inhibition of ET-induced Ca2+ entry appear to be involved.

Involvement of a GTP-binding protein in stimulating action of angiotensin II on calcium channels in vascular smooth muscle cells

The possible involvement of a GTP-binding protein in the regulation of Ca2+ channels by angiotensin II (Ang II) in vascular muscle cells was investigated by the whole-cell voltage-clamp method. Single cells were freshly isolated from guinea pig portal vein. The pipette solution contained high Cs+ to inhibit K+ currents and thereby isolate the Ca2+ channel current. Ba2+ (2 mM) was in the bath solution as a charge carrier for the Ca2+ channel. Application of Ang II (0.1-100 nM) produced an increase in peak amplitude of the Ba2+ current, with a shift of the current-voltage curve in the negative direction. These effects were inhibited by pretreatment with an antagonist of the Ang II receptor, [Sar1,Ile8]-Ang II. Presence of 0.1 mM GTP in the pipette solution stabilized the Ang II action, but 0.3-1.0 mM GDP-beta-S and 1.0 mM GTP-gamma-S inhibited it. GTP-gamma-S alone produced a slowly progressing increase in the basal (unstimulated) current amplitude. Preincubation of muscle tissues with pertussis toxin (1 micrograms/ml, for up to 6 hours at 36 degrees C) or intracellular application of preactivated pertussis toxin (1 micrograms/ml) plus NAD (1 mM) did not inhibit the Ang II action. Cholera toxin (10 micrograms/ml) also had no effect on the Ang II action. These results suggest that the Ang II stimulation of Ca2+ channels in smooth muscle of guinea pig portal vein may be mediated by a G protein that is insensitive to both pertussis toxin and cholera toxin.

Effects of aluminium on electrical and mechanical properties of frog atrial muscle

1. The effects of aluminium on membrane ionic currents were studied in single cardiac myocytes. Most of the work was done on frog atrial cells, but some experiments were also carried out on single cells isolated from rabbit ventricles and atria. 2. The effects of aluminium on the force of contraction of frog atrial trabeculae were also investigated. 3. Aluminium was prepared from AlCl3 as a stock 0.5 M solution which has a pH of 3.5. Before each experiment, this solution was added to the control solution, to give a final concentration of 20-100 micrograms ml-1 aluminium (0.75-3.75 mM AlCl3). The solutions were brought to a pH of 7.4 or 7.6. at which they consist of a mixture of amorphous aluminium hydroxides and a very small amount of soluble ionic aluminium complexes: free aluminium cations (less than 10 pM), aluminohydroxide anions (less than 8 microM). The addition of this suspension reduced the peak inward calcium currents in single rabbit atrial and ventricular cells and in frog atrial cells. In the latter, the peak current was reduced (at + 10 mV) to 45% of control (mean of 9 cells). This effect was reversible upon washout, and was obtained at all membrane potentials, with no shift of the calcium current voltage relationship along the voltage axis. 4. Aluminium also reduced the time-dependent potassium current IK. This reduction was observed at all membrane potentials. For example, at + 10 mV, the mean reduction of IK (n = 9) was to 69% of the control amplitude. This effect, which was very difficult to reverse, was not due to IK rundown. The fully activated current-voltage relationships (obtained by standard 'tail' analysis) showed that the effect of aluminium was due mainly to a decrease in conductance and not to a shift in the activation range of IK. The mean voltage of half activation was shifted by 8 mV in the depolarizing direction (n = 5). 5. The background potassium current IK1 was also slightly but consistently changed in a complex fashion, with an outward shift at membrane potentials positive to -60 mV. For example, at a membrane potential of -40mV, the mean shift was by 22 + 4pA. At more negative potentials, there was an inward shift in the current amplitudes. For example, for steps to -I00 mV the current elicited was larger (more inward) by 53 pA (mean value, n = 10). The reversal potential was slightly shifted (<10 mV) in the hyperpolarizing direction. 6. The force of contraction of frog atrial trabeculae was altered by aluminium in a complex manner, which showed marked seasonal variation. During most of the year, 50-100,ug ml-1 aluminium caused a biphasic change, with an early small and consistent decrease, followed by a large increase in twitch amplitude. For a short period corresponding to the (local) winter months the sensitivity to aluminium was greatly enhanced. Aluminium lOOupgml-1 totally abolished contraction (n = 5), while a lower concentration (20,ug ml- 1) produced a sustained reduction in the force of contraction. Similar biphasic and seasonal responses have been reported to be induced by lanthanum. 7. The biphasic changes in twitch amplitude were independent of the transmembrane sodium gradient. Aluminium produced the same effects when 90% of the extracellular sodium was replaced by lithium. Caffeine (5 mM) attenuated or even inverted the positive inotropic effect of aluminium. These results imply that aluminium alters the release of calcium from intracellular, caffeine-sensitive stores. This could be effected either by augmenting the amount released during each activation, and/or by increasing the loading of stores prior to release.

Effects of captopril on membrane current and contraction in single ventricular myocytes from guinea-pig

1. Single guinea-pig ventricular myocytes were voltage-clamped and cell length was measured with a photodiode array. 2. Captopril (1 x 10(-5) M) reduced both peak early current and active shortening in response to a depolarizing clamp pulse along a similar time course. 3. From a holding potential of around -45 mV peak early inward current was reduced by 37 +/- 9% (P less than 0.001) on exposure to captopril. The early current-voltage relationship was shifted outwards by captopril indicating a reduction in membrane conductance through the L-type calcium channel (ICa). 4. The amplitude of cell shortening in response to depolarizing voltage steps was reduced but the voltage-dependence of contraction after captopril was unchanged. 5. A small negative shift of the potential at which ICa was half-activated was observed after captopril. There was no change in the voltage-dependence of the inactivation variable or in the time-dependence of repriming for ICa. 6. The actions of captopril on ICa and developed shortening were dose-dependent and took place in the same proportion when Ica was increased by isoprenaline. 7. These results are discussed in relation to the effects of captopril on Ica and contraction and to its clinical usage.

Beta-adrenergic and muscarinic regulation of the chloride current in guinea-pig ventricular cells

1. Single guinea-pig ventricular cells were voltage clamped using the patch clamp method combined with the pipette-perfusion technique. The voltage-dependent current systems were mostly blocked, and the background membrane conductance was measured by applying ramp pulses. 2. beta-Adrenergic effectors and related substances such as adrenaline, isoprenaline, forskolin or internal application of cyclic AMP induced a current component which showed a reversal potential near the expected Cl- equilibrium potential as well as an outward rectification in the I-V relation. It is suggested that the activation of this Cl- current was due to phosphorylation of the channel protein or related structure by the cyclic AMP-dependent protein kinase. Coincidentally with the activation of the Cl- current, the membrane capacitance of the cell decreased reversibly. 3. Acetylcholine (ACh) depressed the responses induced by beta-adrenergic stimulation and forskolin, but failed to interfere with the one induced by cyclic AMP. 4. The dose dependence of the Cl- current activation by isoprenaline or forskolin was fitted by the Hill equation, with a coefficient of 1.9 and a half-maximum concentration K 1/2 = 13 nM for isoprenaline, and with a Hill coefficient of 3 and a K 1/2 = 1.2 microM for forskolin. In the presence of 5.5 microM-ACh the dose-response relation shifted to higher doses; K 1/2 was 65 nM for isoprenaline and 3.6 microM for forskolin. 5. Washing out ACh in the presence of isoprenaline frequently caused transient overshoots of the response. When a saturating concentration of isoprenaline was used, this rebound was not observed. 6. The internal application of cyclic GMP enhanced the response of the Cl- current induced by isoprenaline or adrenaline. 7. When cyclic AMP was applied internally, the response was small in most cells. When the cell was superfused with 20 microM-IBMX (3-isobutyl-1-methylxanthine), the Cl- current was consistently induced by the application of cyclic AMP. It is suggested that phosphodiesterase activity strongly buffered the influx of cyclic AMP through the patch pipette tip. 8. We suggest that the compensatory interaction between the beta-adrenergic stimulation and the muscarinic inhibition is at the membrane level, most probably via GTP-binding proteins in activating adenylate cyclase.

Ca2+ channel currents in rat sensory neurones: interaction between guanine nucleotides, cyclic AMP and Ca2+ channel ligands

1. The characteristics have been examined of the high threshold calcium channel current in cultured rat dorsal root ganglion (DRG) neurones recorded in the presence of guanosine-5'-O-(3-thiotriphosphate) (GTP gamma S; 200 microM in the patch pipette). This current, termed IBa, GTP gamma S, was slowly activating and showed little inactivation over 100 ms. 2. External application of forskolin (10 microM) to elevate internal cyclic AMP levels increased the amplitude of IBa, GTP gamma S whereas it had no effect on the control IBa. This cyclic AMP-dependent protein kinase (PKI; 25 microM). 3. The cyclic AMP-dependent phosphorylation induced enhancement of IBa, GTP gamma S was voltage dependent and either did not occur or was observed only transiently at a holding potential (VH) of -30 mV. The forskolin-stimulated enhancement seen at VH -80 mV was lost with a t1/2 of about 1 min when VH was depolarized to -30 mV. Cholera toxin pre-treatment also increased the amplitude of IBa, GTP gamma S at VH -80 mV but not at VH -30 mV. 4. The calcium channel antagonist (-)-202-791 (5 microM) increased the amplitude of IBa, GTP gamma S when applied at VH -80 mV, but either not, or only transiently, at VH -30 mV, as previously observed. This 'agonist' effect of (-)-202-791 was prevented by PKI and was occluded by prior enhancement of IBa, GTP gamma S with forskolin. (-)-202-791 did not increase cyclic AMP levels in DRG neurones. 5. The 'agonist' response of IBa, GTP gamma S to D600 (10 microM) was also occluded by application of forskolin (10 microM) in the patch pipette. Forskolin alone, applied in this manner, increased IBa, GTP gamma S to a similar extent to D600 applied alone. 6. The agonist effect of (+)-202-791 (5 microM) on IBa, GTP gamma S was not prevented by prior enhancement with forskolin, nor was it prevented by PKI. 7. In conclusion, internal GTP gamma S activates G proteins which may interact directly with calcium channels to influence the kinetics of activation and to reduce steady-state inactivation of the channels. There is also an indirect effect on the generation of second messengers such as cyclic AMP. It is likely that forskolin enhances IBa, GTP gamma S by increasing activated Gs coupling to adenylyl cyclase and increasing cyclic AMP generation. The mechanism of action of (-)-202-791 to enhance IBa, GTP gamma S also involves cyclic AMP-dependent phosphorylation.(ABSTRACT TRUNCATED AT 400 WORDS)

Cyclic AMP phosphodiesterases and Ca2+ current regulation in cardiac cells

At least four different isoforms of phosphodiesterases (PDEs) are responsible for the hydrolysis of cAMP in cardiac cells. However, their distribution, localization and functional coupling to physiological effectors (such as ion channels, contractile proteins, etc.) vary significantly among various animal species and cardiac tissues. Because the activity of cardiac Ca2+ channels is strongly regulated by cAMP-dependent phosphorylation, Ca(2+)-channel current (ICa) measured in isolated cardiac myocytes may be used as a probe for studying cAMP metabolism. When the activity of adenylyl cyclase is bypassed by intracellular perfusion with submaximal concentrations of cAMP, effects of specific PDE inhibitors on ICa amplitude are mainly determined by their effects on PDE activity. This approach can be used to evaluate in vivo the functional coupling of various PDE isozymes to Ca2+ channels and their differential participation in the hormonal regulation of ICa and cardiac function. Combined with in vitro biochemical studies, such an experimental approach has permitted the discovery of hormonal inhibition of PDE activity in cardiac myocytes.

The reduction of neuronal calcium currents by ATP-gamma-S is mediated by a G protein and occurs independently of cyclic AMP-dependent protein kinase

We studied the effects of ATP-gamma-S on the T, N and L calcium current components of nodose ganglion neurons using the whole cell variation of the patch clamp technique. ATP-gamma-S can serve as a phosphate donor in kinase-mediated reactions, the donated phosphate group being resistant to the action of phosphatases. We therefore compared the effect of ATP-gamma-S to that of the catalytic subunit of the cyclic AMP-dependent protein kinase (AK-C), included in the recording pipette with 5 mM ATP. AK-C (50 micrograms/ml) had no effect on the T current, and caused a approximately 30% increase in currents containing the N and L components during a 20-min recording, as compared to a approximately 45% decrease in control currents. In contrast, in the presence of 2.5 mM ATP-gamma-S, T currents declined approximately 30%, and currents containing the N and L components declined to a greater extent than control currents, about 65%. In addition, the time to peak current was increased from approximately 14 ms to approximately 40 ms. This effect of ATP-gamma-S on calcium currents was similar to that of certain neurotransmitters or GTP-gamma-S, an activator of G proteins, except that the effects of ATP-gamma-S were delayed 5-7 min relative to GTP-gamma-S. The effects of both ATP-gamma-S and GTP-gamma-S were reduced or abolished in neurons treated with pertussis toxin. We conclude that AK-C regulates neuronal calcium currents, presumably by phosphorylation of channels or associated proteins, and that the ATP-gamma-S-induced reduction of calcium currents cannot be due to its serving as a phosphate donor for endogenous AK.(ABSTRACT TRUNCATED AT 250 WORDS)

Beta-adrenergic regulation of the muscarinic-gated K+ channel via cyclic AMP-dependent protein kinase in atrial cells

Cholinergic and beta-adrenergic stimulations of ionic currents are major physiological mechanisms in the regulation of heart rate and contractility. Muscarinic receptor stimulation is known to reduce beta-adrenergic effects on calcium current via reduction of cyclic AMP. Whether the beta-adrenergic stimulation affects the muscarinic response is not known. I report here that the beta-adrenergic agonist isoproterenol enhanced the muscarinic-activated K+ channel activity in rat atrial cells. Application of cyclic AMP-dependent protein kinase or its catalytic subunit to the cytoplasmic side of the membrane augmented the acetylcholine-activated K+ channel activity twofold to threefold. Increases in channel activity produced by isoproterenol or cyclic AMP-dependent protein kinase were associated with fourfold to fivefold and approximately twofold increases in the mean open and closed time durations, respectively. Alkaline phosphatase treatment reversed these effects. These results suggest that cyclic AMP-dependent phosphorylation of the K+ channel or associated regulatory proteins modulates the gating kinetics of the channel. This mechanism may be important in the regulation of pacemaker activity and, thus, the heart rate during beta-adrenergic stimulation.

Beta-adrenoceptor-mediated depolarization of the resting membrane in guinea-pig papillary muscles: changes in intracellular Na+, K+ and Cl- activities

Effects of beta-adrenergic stimulation on the membrane potential and intracellular Na+, K+ and Cl- activities were examined in isolated guinea-pig ventricular muscles using conventional and ion-selective electrodes. Isoproterenol in concentrations of 30 nM - 1 microM produced a transient depolarization followed by a slight hyperpolarization in electrically stimulated or quiescent papillary muscles. The negative logarithm of the concentration producing 50% maximum effect (pD2) for the membrane-depolarizing effect of isoproterenol was smaller than that for the positive inotropic effect, suggesting that a higher level of cAMP accumulation is required to produce the transient depolarization. Whereas the isoproterenol(1 microM)-induced depolarization was not blocked by tetrodotoxin (10 microM), nifedipine (10 microM), Cs+ (5 mM), Ba2+ (0.3 mM), amiloride (1 mM) or ouabain (10 microM), it was significantly attenuated by anthracene-9-carboxylic acid (1 mM), a Cl(-)-channel blocker. Intracellular K+ activity increased, whereas intracellular Na+ activity slightly decreased during the transient depolarization. Intracellular Cl- activity significantly decreased during the isoproterenol-induced depolarization of the resting membrane. These results suggest that an inward current resulting from outward Cl- movement, rather than inward Na+ movement, may be involved in the beta-adrenoceptor-mediated membrane depolarization.

Cyclic-AMP-dependent phosphorylation modulates the stereospecific activation of cardiac Ca channels by Bay K 8644

Voltage-gated Ca channels have been reported to be regulated by membrane potential, phosphorylation and binding of specific agonists or antagonists such as dihydropyridines. We report here evidence that cyclic AMP (cAMP) modulates the activation of Ca-channel current by the dihydropyridine agonist Bay K 8644. Bay K 8644 (racemate) alone induces a primary voltage-dependent, potentiating effect on peak current amplitude and accelerates the current decay. In contrast, in the presence of cAMP activators, we observed a striking slowing of the decay in addition to the increase in peak current. The agonist (-)-Bay K 8644, but not the antagonist (+)-Bay K 8644, when applied in combination with cAMP, forskolin or isoproterenol, mimics the effect of the racemate. We have interpreted the results presented here in respect of a cAMP-dependent modulation of Bay K 8644 effects on cardiac Ca-channel currents. It may open the new perspective that dephosphorylated and phosphorylated Ca channels have distinct pharmacology.

Actions of the phosphodiesterase inhibitor zardaverine on guinea-pig ventricular muscle

The positive inotropic action of the phosphodiesterase inhibitor zardaverine was investigated in guinea-pig heart muscle. In right papillary muscles, 1-30 microM zardaverine reversibly increased the force of contraction in a concentration-dependent manner. This effect was accompanied by a shortening of contraction and relaxation times. Resting membrane potential was unchanged, whereas action potential amplitude was significantly increased and duration was reduced. In papillary muscles partially depolarised with 22 mM K+, zardaverine (10 and 30 microM) restored slow action potentials, which were not influenced by cimetidine, propranolol or prazosin but were blocked by the calcium channel blocker (+)-nitrendipine or the muscarinic agonist carbachol. cAMP-specific phosphodiesterase III, isolated from guinea-pig ventricular muscle was inhibited by zardaverine as was cAMP-specific phosphodiesterase IV, isolated from dog trachea (IC50s: 0.5 and 0.8 microM, respectively). The results suggest that the observed positive inotropic and electrophysiological effects result from an inhibition of cellular phosphodiesterase.

Calcium channels, potassium channels, and voltage dependence of arterial smooth muscle tone

Resistance arteries exist in a maintained contracted state from which they can dilate or constrict depending on need. In many cases, these arteries constrict to membrane depolarization and dilate to membrane hyperpolarization and Ca-channel blockers. We discuss recent information on the regulation of arterial smooth muscle voltage-dependent Ca channels by membrane potential and vasoconstrictors and on the regulation of membrane potential and K channels by vasodilators. We show that voltage-dependent Ca channels in the steady state can be open and very sensitive to membrane potential changes in a range that occurs in resistance arteries with tone. Many synthetic and endogenous vasodilators act, at least in part, through membrane hyperpolarization caused by opening K channels. We discuss evidence that these vasodilators act on a common target, the ATP-sensitive K (KATP) channel that is inhibited by sulfonylurea drugs. We propose the following hypotheses that presently explain these findings: 1) arterial smooth muscle tone is regulated by membrane potential primarily through the voltage dependence of Ca channels; 2) many vasoconstrictors act, in part, by opening voltage-dependent Ca channels through membrane depolarization and activation by second messengers; and 3) many vasodilators work, in part, through membrane hyperpolarization caused by KATP channel activation.

Increase in calcium channel current by beta-adrenoceptor agonists in single smooth muscle cells isolated from porcine coronary artery

1. The action of catecholamines (isoprenaline and noradrenaline) and forskolin on membrane currents was studied in single cells freshly dispersed from the pig coronary artery by use of the whole-cell clamp method, usually with electrodes containing CsCl. 2. In normal Krebs solution, with and without 30 mM tetraethylammonium (TEA) and 0.5 mM 4-aminopyridine, isoprenaline (1-5 microM) clearly increased the inward currents elicited by membrane depolarization, without affecting the holding current at -80 mV. The same effect was observed when the external Cl- was replaced with isethionate. The outward current recorded with K(+)-containing electrodes was not significantly affected by isoprenaline. 3. In the presence of 67 mM Ba2+ and 30 mM TEA, the maximum inward current recorded with CsCl containing electrodes was 119 +/- 7 pA (the mean +/- s.e.mean, n = 90) in cells where the current was larger than 30 pA. The L-type Ca2+ channel was considered to be responsible for these currents, based on the threshold voltage, the slow time course of decay, the large depolarization necessary to produce inactivation, and the high susceptibility to the Ca2+ channel antagonist, nicardipine. 4. Isoprenaline and noradrenaline increased the amplitude of inward currents evoked by depolarizing pulses. The maximum inward current was potentiated by 43 +/- 7% (n = 12) by isoprenaline and 39 +/- 10% by noradrenaline (n = 6) at a concentration of 1 microM. These effects were strongly inhibited by propranolol, but not phentolamine. Forskolin (10 microM) also potentiated the currents to a similar degree. 5. It is suggested that stimulation of beta 3-adrenoceptors increases the amplitude of inward currents through L-type Ca2 + channels in the pig coronary artery and that intracellular cyclic adenosine monophosphate is likely to be inolved in this action.

Modulation of slow gating process of calcium channels by isoprenaline in guinea-pig ventricular cells

1. The mechanism of enhancement of Ca2+ current by isoprenaline was studied by recording single-channel activity from cell-attached patches on isolated guinea-pig ventricular cells using patch pipettes containing 50 or 100 mM-Ba2+. 2. Isoprenaline (100 nM) increased the amplitude of ensemble average currents by increasing the rate of non-blank sweeps (availability). The current decay during 400 ms steps was significantly slowed by isoprenaline. However, the open probability for the non-blank sweeps elicited by 100 ms steps was only slightly increased by the application of isoprenaline. 3. The durations of the available state (TS) and the unavailable state (TF) were estimated by the number of non-blank and blank sweeps per run, respectively, applying repetitively 100 ms steps at 2 Hz. 4. At large negative holding potentials the distribution of TS was well fitted by an exponential curve, whose time constant was increased from 1.6 to 3.1 sweeps by 100 nM-isoprenaline, while TF distributed approximately single exponentially with a time constant of 2.0 sweeps in control and 1.3 sweeps in the presence of the drug. 5. At depolarized holding potentials a slow voltage-dependent component appeared in the histogram of TF and its time constant was markedly decreased by 100 nM-isoprenaline. 6. The availability-voltage relationship was simulated by the Boltzmann equation with a maximal value of 0.4 in the control. The maximal value was increased to 0.7 and the curve was shifted to a depolarizing direction by 7 mV by 100 nM-isoprenaline. 7. Isoprenaline increased the availability of cardiac Ca2+ channels by increasing the forward rate constant and decreasing the backward rate constant in both voltage-dependent and independent slow state transitions.

Ionic currents contributing to the action potential in single ventricular myocytes of the guinea pig studied with action potential clamp

With the action potential clamp procedure we studied the contribution of various ionic currents to the action potential in single ventricular myocytes. Action potentials were elicited by a current pulse through the suction pipette and recorded by a computer. A representative action potential was then repetitively replayed to the same cell under voltage-clamp conditions. Successive pharmacological blocks of ionic currents allowed for the first time the measurement of the contribution of the L-type calcium current (ICa) and the [Ca2+]i-activated currents as well as the potassium current to the action potential. Experiments using caffeine as a tool to increase calcium release from the sarcoplasmic reticulum supported the idea that INaCa contributes to the plateau during the second half of the action potential and even lasts into diastole, whereas strong elevation of the intracellular [Ca]i during the action potential additionally activated the non-specific cation channel.

Whole-cell calcium current in guinea-pig ventricular myocytes dialysed with guanine nucleotides

1. Whole-cell calcium current (ICa) was recorded in guinea-pig ventricular myocytes superfused with Na+,K(+)-free solution and dialysed with a substrate-free solution (minimum intracellular solution, MICS). A dual tight-seal pipette method was often used to permit pressure-enhanced dialysis of a test solution after a given pre-dialysis. 2. In dual-pipette experiments, test dialysates contained 100 mM-GTP-gamma-S (guanosine 5'-O-(3-thiotriphosphate] or 100 microM-GMP-PNP (guanyl-5'-imidodiphosphate). These non-hydrolysable analogues of guanosine triphosphate (GTP) enhanced ICa amplitude (+ 10 mV) by 20-40%. Dialysates containing 100 microM-GTP or GDP-beta-S (guanosine 5'-O-(2-thiodiphosphate] were ineffective, and pre-dialysis with GDP-beta-S blocked stimulation by GTP-gamma-S. 3. Non-hydrolysable GTP analogues slowed the inactivation of ICa and shifted the voltage eliciting maximum ICa by 5-10 mV in the negative direction. 4. ICa enhancement by GTP analogues was attributed to the activation of three GTP-binding regulatory (G) proteins (Gi, Gp and Gs). In single-pipette experiments, the inactivation of Gi by pre-treatment with pertussis toxin did not block enhancement, and a Gp-activating regimen (external acetylcholine-internal GTP) was without effect. Thus, it is probable that the effects of GTP analogues on ICa were primarily mediated by Gs activation. 5. PI-MICS dialysates contained phosphorylation-pathway inhibitors and were used to inhibit Ca2+ channel phosphorylation via the adenyl cyclase pathway. These were deemed effective since forskolin (1-5 microM) doubled ICa during control dialysis but was without effect after 8 min PI-MICS dialysis. However, 0.1 microM-isoprenaline increased ICa by 35% in myocytes totally unresponsive to forskolin, suggesting that beta-adrenergic receptor occupation can stimulate ICa even when the phosphorylation pathway is blocked. 6. After prolonged dialysis of myocytes with PI-MICS, ICa was still enhanced by pressure-assisted dialysis of 100 microM-GTP-gamma-S or GMP-PNP. We conclude that activated Gs has a direct effect on cardiac Ca2+ channels.

Muscarinic receptor stimulation and cyclic AMP-dependent effects in guinea-pig ventricular myocardium

1. The effect of carbachol on force of contraction, contraction duration, intracellular Na+ activity and cyclic AMP content was studied in papillary muscles of the guinea-pig exposed to isoprenaline or the phosphodiesterase inhibitor 3-isobutyl, 1-methyl xanthine (IBMX). The preparations were obtained from reserpine-pretreated animals and were electrically driven at a frequency of 0.2 Hz. 2. Isoprenaline (10 nM) and IBMX (100 microM) produced comparable positive inotropic effects of 9.8 and 9.7 mN, respectively. Carbachol (3 microM) attenuated the inotropic effects by 82% (isoprenaline) and by 79% (IBMX). The shortening of contraction duration which accompanied the positive inotropic effect of isoprenaline (by 14.9%) and of IBMX (by 22.4%) was not significantly affected by 3 microM carbachol. 3. The positive inotropic effect of 10 nM isoprenaline and of 100 microM IBMX was accompanied by an increase in cellular cyclic AMP content of 58 and 114%, respectively. Carbachol (3 microM) failed to reduce significantly the elevated cyclic AMP content of muscles exposed to either isoprenaline or IBMX. 4. In the quiescent papillary muscle, isoprenaline (10 nM) and IBMX (100 microM) reduced the intracellular Na+ activity by 28 and 17%, respectively. This decline was not influenced by the additional application of 3 microM carbachol. 5. The results demonstrate that muscarinic antagonism in guinea-pig ventricular myocardium exposed to cyclic AMP-elevating drugs is restricted to force of contraction. The underlying mechanism does not apparently involve the cytosolic signal molecule cyclic AMP.

Mechanism of receptor-mediated modulation of the delayed outward potassium current in guinea-pig ventricular myocytes

1. Receptor-mediated modulation of the delayed outward potassium current (IK) was investigated in guinea-pig single ventricular cells by using whole-cell voltage clamp and intracellular dialysis. 2. Isoprenaline increased IK in a dose-dependent manner with a half-maximum dose of 1.8 X 10(-8) M. Isoprenaline (10(-6) M) maximally increased IK by a factor of 2.85. This effect did not depend on the concentration of intracellular Ca2+ [( Ca2+]i). 3. External application of 10(-5) M-forskolin and internal application of 5 X 10(-5) M-cyclic AMP or 5 X 10(-6) M of the catalytic subunit of cyclic AMP-dependent protein kinase (PKA) also increased IK about 3-fold. The effect of isoprenaline on IK was masked by previous application of cyclic AMP. 4. All the above phosphorylating agents increased the amplitude of IK without a significant change in the current kinetics. 5. In the presence of 10(-5) M-forskolin, an additional application of 10(-8) M-12-O-tetradecanoylphorbol-13-acetate, an activator of protein kinase C (PKC), produced a further increase in IK, suggesting that the active sites of PKA and PKC on the IK channel are different. 6. Acetylcholine (10(-6) M) suppressed IK when the current was previously enhanced by 2 X 10(-8) M-isoprenaline, but had little effect in the absence of isoprenaline. 7. We conclude that beta-adrenergic modulation of IK is mediated by cyclic AMP-dependent phosphorylation but not by an increase in [Ca2+]i, that PKA and PKC enhance IK independently, and that acetylcholine antagonizes beta-adrenergic stimulation of IK most probably by inhibiting adenylate cyclase.

G protein coupling of receptors to ionic channels and other effector systems

1. Four questions raised by previous studies that had shown activation of K+ channels by alpha subunits of the type 3 Gi protein are addressed in the present communication: a) are K+ channels specific for one Gi? b) are there more ionic channels under direct G protein control? c) can we confirm using recombinant G alpha s the results obtained with biochemically resolved G alpha s and continue ascribing the regulatory effector to this part of the alpha beta gamma holo-G protein? and d) can we confirm that a single G alpha, Gs alpha in this case, is able to affect more than one type of effector function? 2. We found Gi alpha s are isoforms, that there exist also Gi-insensitive, Go-responsive K+ channels and that G alpha s can be multifunctional. Thus, a single receptor will elicit cellular responses that will depend on the endogenous G protein as well as the type of effector function expressed in it. 3. In another set of experiments we found that G beta gamma s, be they derived from human erythrocytes, human placenta, bovine brain or bovine retina, all inhibit Gk-gated K+ channel activity as seen in inside out membrane patches with GTP as the driving nucleotide. In addition we noted that inhibition was much more effective under basal (no agonist in the pipette) than agonist stimulated conditions, as reported in earlier experiments in which beta-adrenoceptors, Gs and catalytic unit of adenylyl cyclase had been incorporated into phospholipid vesicles. 4. We propose that one of the roles of G beta gamma s in membranes is to quench ligand independent G protein activation by unoccupied receptors. Other roles of G beta gamma s are: a) by re-associating with GDP-G alpha s, to promote interaction with receptors, and b) by dissociating from activated R.G alpha *GTP.beta gamma, to allow for receptor dissociation from GTP-activated G alpha s, which is required to satisfy the catalytic mode of receptor action.

Identification of P2Y purinoceptors associated with voltage-activated cation channels in cardiac ventricular myocytes of the rat

Extracellular ATP has vasodilatory and inotropic effects in the heart. We have demonstrated that extracellular ATP, in a concentration-dependent manner (10 nM-0.1 mM), increased [Ca2+]i in suspensions of isolated fura-2-loaded rat cardiac ventricular myocytes (maximum 96 +/- 10% increase over basal levels, SEM, n = 12, P less than 0.01). The increase in [Ca2+]i was often biphasic, with an initial fast phase (less than 1 s) of low amplitude, followed by a slower phase of higher amplitude. A second application of ATP had little effect, and ATP abolished the effect of subsequent electrical stimulations, even through the cells were still able to respond with an increase in [Ca2+]i to KCl-induced depolarization or stimulation by caffeine. Pretreatment of cells with nifedipine, verapamil, caffeine, ryanodine, or 8-(N,N-diethylamino)octyl 3,4,5-trimethoxybenzoate hydrochloride attenuated the effect of extracellular ATP on [Ca2+]i, and binding of extracellular free calcium by excess EGTA completely abolished the effects of extracellular ATP and electrical stimulation. Extracellular ATP increased bisoxonol fluorescence in ventricular myocytes, indicating depolarization of the sarcolemma. Pretreatment of the myocytes with tetrodotoxin (50 microM), or replacement of NaCl in the incubation buffer with the impermeant cation N-methyl-D-glucamine, suppressed the extracellular ATP effect on [Ca2+]i. ADP and AMP had smaller effects on [Ca2+]i than ATP; adenosine had no effect. ATP analogues showed the following rank order of potency in increasing [Ca2+]i or bisoxonol fluorescence: ATP greater than or equal to 2-methylthioATP much greater than adenosine 5'-O-[3-thio]triphosphate greater than adenosine 5'-[alpha, beta-methylene]triphosphate approximately adenosine 5'-[beta, gamma-methylene]triphosphate approximately adenosine 5'-[beta, gamma-imino]triphosphate greater than adenosine. These data are consistent with the presence of purinoceptors (P2Y subtype) on the sarcolemma of cardiac ventricular myocytes of the rat, which upon activation lead to depolarization and activation of cation channels of the sarcolemma and flux of extracellular Ca2+ into the cells. This may result in further flux of Ca2+ into the cytosol from intracellular stores. The effects of extracellular ATP on [Ca2+]i in rat cardiac ventricular myocytes may, in part, explain the direct inotropic effects of extracellular ATP on the mammalian heart.

Thyroid hormone modulates membrane currents in guinea-pig ventricular myocytes

Thyroid hormones have been previously shown to alter cardiac electrophysiological and mechanical properties in humans and in experimental animals. To investigate electrophysiological mechanisms responsible for some of these alterations, we recorded action potentials and membrane currents from isolated ventricular myocytes obtained from euthyroid, hypothyroid and hyperthyroid guinea-pigs. Hyperthyroidism was induced by injecting 150 micrograms/kg triiodothyronine for 8-11 days, and hypothyroidism was induced by propylthiouracil treatment for 35-45 days. We found that the slow inward current, was increased by hyperthyroidism and decreased by hypothyroidism: in euthyroid, hyperthyroid and hypothyroid myocytes peak slow inward current was (mean +/- SEM): -1.08 +/- 0.06 nA, -1.83 +/- 0.18a nA and -0.64 +/- 0.07a nA, respectively (a, p less than 0.005). In addition, the membrane potential at which peak slow inward current occurred was modified by the thyroid state and in euthyroid, hyperthyroid and hypothyroid myocytes it was (mean +/- SEM): 4.8 +/- 0.7 mV, -1.8 +/- 1.6a mV and 11.0 +/- 1.4a mV, respectively. The outward rectifying current, was also affected by the thyroid state, and in euthyroid, hyperthyroid and hypothyroid myocytes, the amplitude at VM = +60 mV was (mean +/- SEM): 0.51 +/- 0.09 nA, 1.15 +/- 0.08a nA and 0.49 +/- 0.05 nA, respectively. a, p less than 0.001 compared to euthyroid myocytes. Intraperitoneal administration of a single dose of triiodothyronine to guinea-pigs, 2 h prior to the electrophysiological experiment, increased the slow inward current amplitude, as was seen with chronic hyperthyroidism, but had no significant effect on the outward current and on the action potential.(ABSTRACT TRUNCATED AT 250 WORDS)

Interactive effects of isoprenaline, forskolin and acetylcholine on Ca2+ current in frog ventricular myocytes

1. Calcium currents (ICa) were measured in single cells isolated from frog ventricle using the whole-cell patch-clamp technique and a perfused pipette. The dose-dependent stimulatory effects of isoprenaline (Iso, 0.1-100 microM) and forskolin (Fo. 0.1-50 microM) on ICa were determined in the presence and absence of acetylcholine (ACh, 10 microM) and/or threshold concentrations of Fo (0.2 microM) and Iso (0.05 microM), respectively. EC50 (i.e. concentration of Iso or Fo at which the response was 50% of the maximum) and Emax (i.e. maximal stimulation of Ica expressed as percentage increase in ICa with respect to control) were measured under each condition. 2. ACh increased EC50 for the stimulatory action of Iso on ICa from 0.84 to 3.72 microM while it reduced Emax from 658 to 185%. Thus, ACh mainly reduced the efficacy of Iso to stimulate ICa. 3. ACh increased EC50 for the stimulatory action of Fo on ICa from 2.06 to 10.26 microM but only slightly reduced Emax from 893 to 778%. Thus, ACh mainly reduced the potency of Fo to stimulate ICa. 4. Intracellular perfusion with 100 microM of hydrolysis-resistant GTP analogues, GTP-gamma-S [guanosine-5'-O-(3-thiotriphosphate)] and Gpp (NH)p (5'-guanylylimido-diphosphate), had no effect on basal ICa but reduced by greater than 50% the stimulatory effect of 2 microM-Iso on ICa. 5. In the presence of Gpp(NH)p or GTP-gamma-S, Fo (3 microM) reversibly increased ICa by 490%, as compared to a 717% increase in control (GTP) intracellular solution. Although ACh could still inhibit Fo-stimulated ICa, the degree of inhibition was significantly smaller than in the presence of GTP. 6. Extracellular perfusion with low concentrations of a combination of Iso (33 nM) and Fo (330 nM) enhanced ICa to a much greater extent than did either agent alone at 3 times higher concentrations. Thus, low concentrations of Iso and Fo appear to increase ICa in a synergistic fashion. 7. ICa stimulated by a combination of Iso and Fo appeared to be more resistant to inhibition by ACh than when stimulated by either alone. It was the efficacy, rather than the potency, of ACh to inhibit ICa that was reduced upon dual stimulation of ICa. 8. In the presence of 0.2 microM-Fo, EC50 and Emax for the effects of Iso on ICa were 0.27 microM and 619%, respectively. By comparison with the effects of Iso alone, Fo reduced EC50 approximately 3 times with no significant change in maximal stimulation.(ABSTRACT TRUNCATED AT 400 WORDS)

Regulation of Ca2+ current in frog ventricular myocytes by the holding potential, c-AMP and frequency

The whole-cell patch-clamp technique was used to study the effects of holding potential and frequency on the Ca2+ current in frog ventricular myocytes. INa was blocked by TTX, and ica was activated with depolarizing clamps from different holding potentials. Variation of the holding potential revealed three new effects on ica: (1) At -40 mV iCa declined with a time constant of 15 min, while at -90 mV, this irreversible decline (run down) in iCa did not occur. (2) The decline of iCa at -40 mV was biphasic: run down was preceeded by a slow inactivation with a time constant of 40 s, which was reversible upon returning the holding potential to -90 mV. (3) Increasing the frequency of the clamp pulses from 0.1 to 1 Hz led to a rapid decline of iCa when the holding potential was positive to -60 mV, but at -90 mV had either no effect or increased iCa by 35%, if c-AMP was included in the dialyzing solution. On the other hand, c-AMP did not alter the time course of the run down and the slow inactivation. Replacement of extracellular Ca2+ by Ba2+ markedly slowed iCa kinetics, but did not change the very slow inactivation or the frequency-induced enhancement of iCa. Injection of c-AMP led to a transient increase of iCa. The phosphodiesterase inhibitor theophylline enhanced the amplitude of the transient and slowed its decay. This effect was mimicked by increased frequency. It is concluded that frequency-induced enhancement of iCa is highly dependent on the holding potential, independent of Ca2+, and may involve elevation of the intracellular level of c-AMP via inhibition of phosphodiesterase activity. The new type of very slow inactivation is probably under direct voltage control and independent of Ca2+ and c-AMP.

Chloride conductance regulated by cyclic AMP-dependent protein kinase in cardiac myocytes

In heart cells, cyclic AMP-dependent protein kinase (PKA) regulates calcium- and potassium-ion current by phosphorylating the ion channels or closely associated regulatory proteins. We report here that isoprenaline induced large chloride-ion currents in voltage-clamped, internally-dialysed myocytes from guinea-pig ventricles. The Cl- current could be activated by intracellular dialysis with cAMP or the catalytic subunit of PKA, indicating regulation by phosphorylation. In approximately symmetrical solutions of high Cl- concentration, the macroscopic cardiac Cl- current showed little rectification, unlike the single-channel current in PKA-regulated Cl- channels of airway epithelial cells. But, like epithelial Cl- -channel currents, the cardiac Cl- current was sensitive to the distilbene,4,4'-dinitrostilbene-2,2'-disulphonic acid (DNDS). In the absence of kinase activation, cardiac sarcolemmal Cl- conductance was negligible. During beta-adrenergic stimulation of the heart, this novel Cl- conductance should accelerate action-potential repolarization and so protect impulse propagation in the face of the possibly arrhythmogenic increases in heart rate and in calcium entry into the cells.

Modulation of the delayed rectifier potassium current in frog cardiomyocytes by beta-adrenergic agonists and magnesium

1. The regulation of IK and ICa were studied in single cells isolated from bull-frog atrium using the whole-cell configuration of the patch clamp and a perfused patch pipette. 2. IK was increased approximately 50-100% and ICa was increased approximately 6-10 times by 1 microM-isoprenaline, 5 microM-forskolin, or internal perfusion with 30 microM-cyclic AMP. The effects of cyclic AMP and isoprenaline were not additive. The shape of the concentration-response curves and the EC50 values for the effects of cyclic AMP on ICa and on IK were very similar (2.3 microM for IK and 1.7 microM for ICa). 3. Elevation of intracellular cyclic AMP had a similar effect on IK regardless of whether ICa was blocked with Cd2+ or not. Increasing ICa with dihydropyridine Ca2+ channel agonists had no effect on IK amplitude. 4. Isoprenaline or cyclic AMP caused an increase in the fully-activated IK and also shifted the activation curves to more negative potentials in most cells. The shift in the activation curve was reversible and was also observed when ICa was blocked with Cd2+. The rate of activation of IK was increased and the rate of deactivation of IK was slowed by isoprenaline. 5. After breaking the membrane patch and initiating whole-cell recording, IK ran down with time in about 50% of the cells examined when the intracellular solution contained 1 mM [Mg2+]. In contrast, when the solution contained 0.3 mM [Mg2+], rundown was almost never observed. Internal perfusion with increasing concentrations of [Mg2+] caused reversible decreases in the maximum amplitude of IK and shifted the IK activation curve slightly to more negative potentials, but had negligible effects upon the shape or the curvature of the fully activated current-voltage relationship.

Modulation of single slow (L-type) calcium channels by intracellular ATP in vascular smooth muscle cells

Involvement of ATP in the regulation of slow (L-type) Ca2+ channels of vascular smooth muscle cells was investigated by recording single Ca2+ channel currents (single-channel conductance of 18 pS) using a patch clamp technique. In the cell-attached configuration, intracellular composition was modified by permeabilizing the cell membrane with mechanical disruption at one end of the cell. Single cells were freshly isolated from guinea-pig portal vein by collagenase treatment. For the channel recordings, the pipette solution contained 100 mM Ba2+ and the bath contained K+-rich solution (with 5 mM EGTA) to depolarize the membrane to near 0 mV. The channel activity decreased usually within 3 min after permeabilizing the cell end and exposure to ATP-free bath solution. If ATP (1-5 mM) was applied to the bath (access to cell interior) before complete disappearance of channel activity, channel activity was partially recovered. ATP did not change the current amplitude (i) or the mean open time of the channels, whereas the number of channels available for opening and/or the probability of their being open (NPo) were increased by ATP. A non-hydrolyzable analogue of ATP, AMP-PNP, did not exert an ATP-like effect; ATP-gamma-S had a weak effect. With 1 microM Bay-K-8644 (Ca2+ channel agonist) in the pipette, the activity of the Ca2+ channel was high; such activity persisted for more than 10 min after permeabilizing the cell and exposing to ATP-free solution containing KCN (1 mM) and 2-deoxy-D-glucose (10 mM).(ABSTRACT TRUNCATED AT 250 WORDS)

Autonomic regulation of a chloride current in heart

In isolated heart cells, beta-adrenergic receptor stimulation induced a background current that was suppressed by simultaneous muscarinic receptor stimulation. Direct activation of adenylate cyclase with forskolin also elicited this current, suggesting regulation by adenosine 3',5'-monophosphate (cAMP). This current could be recorded when sodium, calcium, and potassium currents were eliminated by channel antagonists or by ion substitution. Alteration of the chloride equilibrium potential produced changes in the reversal potential expected for a chloride current. Activation of this chloride current modulated action potential duration and altered the resting membrane potential in a chloride gradient-dependent manner.

Nonstationary fluctuation analysis of the delayed rectifier K channel in cardiac Purkinje fibers. Actions of norepinephrine on single-channel current

We have studied the large increase in macroscopic potassium channel current caused by catecholamines in mammalian cardiac cells. An increase in macroscopic K current could result from either an increase in the single-channel current or by an increase in the number of channels that are open. Therefore, we have measured nonstationary potassium current fluctuations under voltage clamp conditions to determine whether norepinephrine increases the current through this channel. The single-channel current (at a potential of -30 mV in 4 mM external [K]) was estimated to be 3.7 pA and was not altered by concentrations of norepinephrine up to 2 microM. The spectral density of the current fluctuations were fitted well by a sum of 2 Lorentzians with corner frequencies that correspond with the measured time constants for deactivation of the macroscopic K current tails. We conclude that the increase in macroscopic K current caused by norepinephrine in these cells is not the result of an increase in single-channel conductance and therefore must involve an increase in the number of open K channels.

Cytosolic magnesium modulates calcium channel activity in mammalian ventricular cells

The effect of cytosolic free Mg2+ concentration on the regulation of myocardial function was studied by dialyzing isolated guinea pig ventricular myocytes with different internal Mg2+ concentrations [( Mg2+]i). We found that elevation of [Mg2+]i shortened the action potential and suppressed the Ca2+ current. Mean values recorded for action potential duration in cells dialyzed with solutions containing 0, 1.3, and 9.4 mM Mg2+ were 620 +/- 40, 400 +/- 25, and 60 +/- 10, respectively. The suppressive effect of [Mg2+]i on the action potential duration correlated significantly with the suppressive effects of [Mg2+]i on the Ca2+ current. In cells dialyzed with nominally zero Mg2+, calcium current was prominent (3.5 +/- 0.58 nA). At [Mg2+]i of 1.4 mM, calcium current was significantly smaller than in zero [Mg2+]i and was almost completely inhibited by dialysis of the cell with 9.4 mM Mg2+. The Mg2+-induced block of the Ca2+ current was due to steady-state inactivation of the high threshold calcium channel. The block was observed in the presence or absence of adenosine 3',5'-cylic monophosphate and was not reversed by elevation of external Ca2+ concentration, addition of adrenaline, or large negative potentials. These data suggest that cytosolic Mg2+ regulates Ca2+ channel activity by a novel mechanism, unrelated to its effect as a blocking particle of the open channel.