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

Regulation of cardiac L-type Ca²⁺ channel CaV1.2 via the β-adrenergic-cAMP-protein kinase A pathway: old dogmas, advances, and new uncertainties

In the heart, adrenergic stimulation activates the β-adrenergic receptors coupled to the heterotrimeric stimulatory Gs protein, followed by subsequent activation of adenylyl cyclase, elevation of cyclic AMP levels, and protein kinase A (PKA) activation. One of the main targets for PKA modulation is the cardiac L-type Ca²⁺ channel (CaV1.2) located in the plasma membrane and along the T-tubules, which mediates Ca²⁺ entry into cardiomyocytes. β-Adrenergic receptor activation increases the Ca²⁺ current via CaV1.2 channels and is responsible for the positive ionotropic effect of adrenergic stimulation. Despite decades of research, the molecular mechanism underlying this modulation has not been fully resolved. On the contrary, initial reports of identification of key components in this modulation were later refuted using advanced model systems, especially transgenic animals. Some of the cardinal debated issues include details of specific subunits and residues in CaV1.2 phosphorylated by PKA, the nature, extent, and role of post-translational processing of CaV1.2, and the role of auxiliary proteins (such as A kinase anchoring proteins) involved in PKA regulation. In addition, the previously proposed crucial role of PKA in modulation of unstimulated Ca²⁺ current in the absence of β-adrenergic receptor stimulation and in voltage-dependent facilitation of CaV1.2 remains uncertain. Full reconstitution of the β-adrenergic receptor signaling pathway in heterologous expression systems remains an unmet challenge. This review summarizes the past and new findings, the mechanisms proposed and later proven, rejected or disputed, and emphasizes the essential issues that remain unresolved.

Biphasic Response of Action Potential Duration to Sudden Sympathetic Stimulation in Anesthetized Cats

Although certain roles of the sympathetic nervous system have been suggested as possible mechanisms of life-threatening arrhythmias and sudden cardiac death, the dynamic electrophysiological response to sympathetic activation remains unclear. The aim of this study was to investigate the dynamic response of action potential duration (APD) to sudden sympathetic stimulation (SYM) using monophasic action potential (MAP) recording. In 10 anesthetized cats, MAPs were continuously recorded from the right ventricular endocardium under constant pacing. The dynamic response of the APD to SYM (3 Hz) were examined before and after the administration of propranolol (0.5 mg/kg i.v.) (n=5) or phentolamine (1.0 mg/kg i.v.) (n=5). In response to SYM, the APD was transiently prolonged by 5.5+/-3.2 ms at 7.0+/-1.3 s, and monotonically shortened toward a steady-state level (-14.5+/-6.9 ms). Propranolol almost abolished both the transient prolongation (6.6+/-4.5 to 0.2+/-0.4 ms, p<0.05) and the steady-state shortening (-13.7+/-3.6 to -1.1+/-2.4 ms, p<0.005), whereas phentolamine did not have a significant effect on the response of APD to SYM. These findings might partly account for the propensity of ventricular arrhythmias to occur immediately after sudden sympathetic activation.

Aplysia cardioactive peptide (NdWFamide) enhances the L-type Ca2+ current of Aplysia ventricular myocytes

NdWFamide is a D-amino acid containing tripeptide purified from Aplysia heart. Although the cardioexcitatory action of NdWFamide is well established, little is known about how the excitatory action is induced. To examine the action of the peptide on the ion channels expressed in the Aplysia heart muscles, we carried out whole cell clamp experiments in the isolated Aplysia ventricular myocytes. We found that the high voltage-activated (HVA) Ca(2+) current of Aplysia ventricular myocytes is mostly a nifedipine-sensitive L-type current, and that the current was enhanced by NdWFamide via the activation of G proteins.

Regulation of cardiac and smooth muscle Ca(2+) channels (Ca(V)1.2a,b) by protein kinases

High voltage-activated Ca(2+) channels of the Ca(V)1.2 class (L-type) are crucial for excitation-contraction coupling in both cardiac and smooth muscle. These channels are regulated by a variety of second messenger pathways that ultimately serve to modulate the level of contractile force in the tissue. The specific focus of this review is on the most recent advances in our understanding of how cardiac Ca(V)1.2a and smooth muscle Ca(V)1.2b channels are regulated by different kinases, including cGMP-dependent protein kinase, cAMP-dependent protein kinase, and protein kinase C. This review also discusses recent evidence regarding the regulation of these channels by protein tyrosine kinase, calmodulin-dependent kinase, purified G protein subunits, and identification of possible amino acid residues of the channel responsible for kinase regulation.

Genetic analysis of the Drosophila Gs(alpha) gene

One of the best understood signal transduction pathways activated by receptors containing seven transmembrane domains involves activation of heterotrimeric G-protein complexes containing Gs(alpha), the subsequent stimulation of adenylyl cyclase, production of cAMP, activation of protein kinase A (PKA), and the phosphorylation of substrates that control a wide variety of cellular responses. Here, we report the identification of "loss-of-function" mutations in the Drosophila Gs(alpha) gene (dgs). Seven mutants have been identified that are either complemented by transgenes representing the wild-type dgs gene or contain nucleotide sequence changes resulting in the production of altered Gs(alpha) protein. Examination of mutant alleles representing loss-of-Gs(alpha) function indicates that the phenotypes generated do not mimic those created by mutational elimination of PKA. These results are consistent with the conclusion reached in previous studies that activation of PKA, at least in these developmental contexts, does not depend on receptor-mediated increases in intracellular cAMP, in contrast to the predictions of models developed primarily on the basis of studies in cultured cells.

Serine/threonine protein phosphatases and synaptic inhibition regulate the expression of cholinergic-dependent plateau potentials

We previously identified cholinergic-dependent plateau potentials (PPs) in CA1 pyramidal neurons that were intrinsically generated by interplay between voltage-gated calcium entry and a Ca(2+)-activated nonselective cation conductance. In the present study, we examined both the second-messenger pathway and the role of synaptic inhibition in the expression of PPs. The stimulation of m1/m3 cholinergic receptor subtypes and G-proteins were critical for activating PPs because selective receptor antagonists (pirenzepine, hexahydro-sila-difenidol hydrochloride, 4-diphenylacetoxy-N-methylpiperidine methiodide) and intracellular guanosine-5'-O-(2-thiodiphosphate) prevented PP generation in carbachol. Intense synaptic stimulation occasionally activated PPs in the presence of oxytremorine M, a cholinergic agonist with preference for m1/m3 receptors. PPs were consistently activated by synaptic stimulation only when oxytremorine M was combined with antagonists at both GABA(A) and GABA(B) receptors. These latter data indicate an important role for synaptic inhibition in preventing PP generation. Both intrinsically generated and synaptically activated PPs could not be elicited following inhibition of serine/threonine protein phosphatases by calyculin A, okadaic acid, or microcystin-L, suggesting that muscarinic-induced dephosphorylation is necessary for PP generation. PP genesis was also inhibited following irreversible thiophosphorylation by intracellular perfusion with ATP-gamma-S. These data indicate that the expression of cholinergic-dependent PPs requires protein phosphatase-induced dephosphorylation via G-protein-linked m1/m3 receptor(s). Moreover, synaptic inhibition via both GABA(A) and GABA(B) receptors normally prevents the synaptic activation of PPs. Understanding the regulation of PPs should provide clues to the role of this regenerative potential in both normal activity and pathophysiological processes such as epilepsy.

Regulation of cardiac L-type Ca2+ channel by coexpression of G(alpha s) in Xenopus oocytes

Activation of G(alpha s) via beta-adrenergic receptors enhances the activity of cardiac voltage-dependent Ca2+ channels of the L-type, mainly via protein kinase A (PKA)-dependent phosphorylation. Contribution of a PKA-independent effect of G(alpha s) has been proposed but remains controversial. We demonstrate that, in Xenopus oocytes, antisense knockdown of endogenous G(alpha s) reduced, whereas coexpression of G(alpha s) enhanced, currents via expressed cardiac L-type channels, independently of the presence of the auxiliary subunits alpha2/delta or beta2A. Coexpression of G(alpha s) did not increase the amount of alpha1C protein in whole oocytes or in the plasma membrane (measured immunochemically). Activation of coexpressed beta2 adrenergic receptors did not cause a detectable enhancement of channel activity; rather, a small cAMP-dependent decrease was observed. We conclude that coexpression of G(alpha s), but not its acute activation via beta-adrenergic receptors, enhances the activity of the cardiac L-type Ca2+ channel via a PKA-independent effect on the alpha1C subunit.

Effects of a novel, potent benzothiazepine Ca2+ channel antagonist, DTZ323, on guinea-pig ventricular myocytes

The effects of a 1,5-benzothiazepine derivative, (+)-cis-3-(acetyloxy)-5-[2-[[2-(3,4-dimethoxyphenyl)ethyl]-methyla mino]ethyl]-2,3-dihydro-2-(4-methyoxyphenyl)-1,5-benzothiazepine-4 (5H)-one (DTZ323), on membrane currents were investigated in guinea-pig ventricular myocytes using the whole-cell patch-clamp technique. DTZ323 suppressed the L-type Ca2+ channel currents (I[Ca(L)]) more selectively than the T-type Ca2+ channel and the Na+ channel currents. DTZ323 inhibited I[Ca(L)] in a use- and a voltage-dependent manner with 24 times higher potency than that of diltiazem. Rate of recovery of I[Ca(L)] from the conditioned block by DTZ323 was faster compared with diltiazem and verapamil, and was steeply dependent on the holding potential at resting membrane potential range in ventricular myocytes (-90 to -60 mV). Our results suggest that DTZ323 is a selective Ca2+ channel antagonist, the most potent among the 1,5-benzothiazepine Ca2+ channel antagonists, and that the voltage- and use-dependent effect of DTZ323 on I[Ca(L)] is due to the steep voltage dependence of the rate of dissociation from the cardiac L-type Ca2+ channels.

Facilitation of T-type calcium current in bullfrog atrial cells: voltage-dependent relief of a G protein inhibitory tone

1. The properties of the low-threshold calcium current, ICa,T, were investigated in bullfrog isolated atrial cardiomyocytes using the whole-cell, patch-clamp technique under control conditions and during beta-adrenergic stimulation. 2. The intracellular application of GTP gamma S or adenosine-5'-O-3-thiotriphosphate (ATP gamma S), poorly hydrolysable analogues of GTP and ATP, respectively, barely affected ICa,T amplitude in control conditions. beta-Adrenergic stimulation effects were more marked in the presence of ATP gamma S. 3. The intracellular application of GDP beta S and ADP reduced ICa,T amplitude. In cells pretreated with pertussis toxin, ICa,T amplitude was significantly increased. In both conditions, the addition of isoprenaline was without effect. 4. Under both control and beta-adrenergic-stimulated conditions, a conditioning prepulse to +70 mV did not fully inactivate ICa,T; rather ICa,T facilitation often occurred after beta-adrenergic stimulation. 5. In GTP gamma S- and ATP gamma S-dialysed cells, ICa,T facilitation was generally observed after a prepulse; it was larger in the ATP gamma S dialysis. Facilitation was sustained but ended immediately upon cessation of conditioning prepulses. After beta-adrenergic stimulation, facilitation was more marked in GTP gamma S- than in ATP gamma S-dialysed cells. 6. ICa,T facilitation was prevented by the intracellular application of GDP beta S and by pertussis toxin pretreatment. 7. ICa,T facilitation developed markedly in the presence of intracellular cyclic AMP. This effect was prevented by pertussis toxin pretreatment of the cells. 8. It is thus proposed that ICa,T is under a double antagonistic control by both a Gs and a Gi protein. Furthermore, the double-pulse-induced facilitation of ICa,T results from a voltage-dependent relief of the Gi protein inhibitory tone. Such an effect is increased by protein kinase A-dependent phosphorylation, presumably of the Gi protein.

Stable guanosine 5'-triphosphate-analogues inhibit specific (+)-[3H]isradipine binding in rat hearts by a Ca(2+)-lowering, G protein-independent mechanism

We investigated if and how stable guanosine 5'-triphosphate-analogues affect (+)-[3H]isradipine binding in rat hearts. Gpp(NH)p and GTP-gamma-S inhibit specific (+)-[3H]isradipine binding in membranes and cell-homogenates by reducing the binding density without changing the Kd of the k-1. Inhibition by Gpp(NH)p was less in crude tissue homogenates than in membranes apparently due to a soluble factor. Pretreatment of cardiomyocytes with cholera toxin or the presence of the protein kinase A inhibitor, PKI6-22, did not influence the effect of 10(-3) M Gpp(NH)p on binding. The inhibitory effect of 10(-3) M Gpp(NH)p was not significantly altered in membranes from in vivo pertussis toxin treated rats. The addition of 10(-3) M Ca2+ or Mg2+ abolished the inhibitions. Gpp(NH)p in the concentration that inhibits binding, reduced the free concentration of Ca2+. The Ca(2+)-lowering effect of 10(-3) M Gpp(NH)p produced 70%, 60% and 100% of the inhibition in membranes, sonicated and unsonicated cell homogenates. Thus, Gpp(NH)p inhibited specific (+)-[3H] isradipine binding mainly by lowering the free concentration of Ca2+ by chelation and not by activation of cholera toxin or pertussis toxin-sensitive G proteins or protein kinase A.

Developmental changes in the actions of phosphatase inhibitors on calcium current of rabbit heart cells

We used whole-cell voltage clamp to compare the modulation of calcium current density (ICa, picoampere per picofarad) of freshly isolated, adult and newborn rabbit heart in response to intracellular application of microcystin and okadaic acid, both of which block phosphatase activity of phosphatase type 1 and 2A. Newborn cells showed a much larger response to the intracellular application of either microcystin or okadaic acid than did adult cells. In newborn cells, the application of microcystin produced an increase in ICa which appeared to maximize ICa, as shown by the rise in ICa to levels which could be reached by application of 10 microM forskolin or by the intracellular application of 200 microM 3',5'-cyclic adenosine monophosphate (cAMP). In adult cells, the maximal response to microcystin was considerably less than that obtainable with forskolin or cAMP. After achieving a maximal response with microcystin, the addition of forskolin increased ICa further in adult cells but elicited no additional response in newborn cells. The treatment of cells with 0.1 microM isoproterenol, a concentration approximately equal to that required for a half-maximal response, strongly potentiated the effect of microcystin in newborn cells, but not in adult cells. We propose that newborn rabbit heart cells compared with adult rabbit heart cells have a greater level of protein phosphatase activity (perhaps combined with a somewhat greater kinase activity), a greater proportion of the protein phosphatase activity in the form of protein phosphatase type 1 (which is inhibited by isoproterenol) and a greater dependence on the inhibition of protein phosphatase as a mechanism of action of isoproterenol, compared with the increase in kinase activity on calcium channels.

Modulation of cardiac L-type Ca2+ channels by GTP gamma S in response to isoprenaline, forskolin and photoreleased nucleotides

1. Using the patch-clamp recording technique, we have investigated the effects of chronic intracellular application of guanosine thiotriphosphate (GTP gamma S) by cell dialysis, on the potentiation of L-type Ca2+ currents (ICa) by isoprenaline and forskolin and also by GTP gamma S and cyclic AMP released intracellularly by flash-photolysis of their caged derivatives. 2. GTP gamma S prevented enhancement of ICa by isoprenaline with an IC50 of approximately 10 microM and considerably reduced the ability of forskolin to increase ICa. In addition GTP gamma S also reduced the time-to-peak response for potentiation of ICa by forskolin. Responses to forskolin were abolished by co-dialysis of cells with the cyclic AMP antagonist, Rp-adenosine-3'-5'-mono-thionophosphate (Rp-cAMPS). 3. Photoreleased GTP gamma S (PR-GTP gamma S; approximately 23 microM) generally induced a biphasic increase in ICa. This response was also inhibited by chronic intracellular dialysis with GTP gamma S with an IC50 of approximately 1 microM. 4. Pretreatment of cells with pertussis toxin (PTX) reversed the inhibitory effect of 100 microM GTP gamma S on isoprenaline-induced stimulation of ICa. However, PTX pretreatment did not restore the activating action of PR-GTP gamma S inhibited by chronic application of GTP gamma S. 5. Photoreleased cyclic AMP (approximately 5 microM; PR-cyclic AMP) increased peak ICa. This effect was inhibited by dialysis of cells with Rp-cAMPS and by stimulation of ICa by the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine. Co-dialysis of cells with uncaged GTP gamma S reduced the time-to-peak for PR-cyclic AMP mediated activation of ICa but did not affect the magnitude of the response. 6. It is concluded that chronically applied GTP gamma S can (i) inhibit activation of ICa by isoprenaline by interacting with a PTX-sensitive guanosine nucleotide binding (G-) protein located upstream of adenylate cyclase (possibly Gi) and (ii) accelerate the response to cyclic AMP dependent phosphorylation possibly by interacting with a G-protein coupled directly to the channel. 7. In view of this diverse range of effects, care should be taken when using GTP gamma S to characterize G-protein-mediated events, since the resulting physiological response may be due to activation of several G-protein containing pathways.

Mechanisms of beta-adrenergic stimulation of cardiac Ca2+ channels revealed by discrete-time Markov analysis of slow gating

Individual cardiac Ca2+ channels cycle slowly between a mode of gating in which the channel is available to open, and one in which the channel remains silent. The regulation of this multisecond cycling process by isoproterenol was investigated by single-channel recording and the development of a discrete-time Markov model that describes the slow switching among modes in terms of (de) phosphorylation reactions. The results provide evidence that isoproterenol increases Ca2+ channel activity by a reciprocal regulatory mechanism: not only is the phosphorylation rate of the channel increased, but also the dephosphorylation rate decreased. The discrete-time Markov formalism should prove useful as a general tool for understanding the mode switching demonstrated by a number of ionic channels.

A comparative analysis of the time course of cardiac Ca2+ current response to rapid applications of beta-adrenergic and dihydropyridine agonists

A fast perfusion system was used to analyze the kinetics of the response of L-type calcium current (ICa) to rapid exposures to beta-adrenergic or dihydropyridine agonists in whole-cell patch-clamped frog ventricular myocytes. The perfusion system was based on the lateral motion of an array of plastic capillary tubes from which solutions flowed at a velocity of approximately 5 cm/s. Movement from one capillary to the adjacent one occurred in < 20 ms and complete exchange of extracellular solution was achieved in < 50 ms as demonstrated by the block of ICa by fastflow application of Cd during a depolarizing pulse. Fastflow applications of increasing concentrations of isoprenaline (Iso) led to a dose-dependent stimulation of ICa at [Iso] > 1 nM. The response of ICa to Iso always started after a delay of several seconds. The delay duration decreased as [Iso] increased, and was typically approximately 3 s at 10 microM Iso. The rising phase of ICa increase was monophasic and independent of [Iso] > 100 nM. For short applications of Iso (8.8 s), half maximal and maximal stimulation of ICa occurred approximately 20 s and approximately 40 s after the beginning of Iso application, respectively. When Iso was applied during a depolarizing pulse (with Ba as the charge carrier), IBa never increased during that pulse. The kinetics of the ICa response to Iso were not affected by varying the voltage clamp protocols or the ionic composition of intracellular and extracellular solutions. In comparison with the effects of Iso, the stimulatory effect of the dihydropyridine agonist (-)Bay K 8644 on ICa was approximately 15 times faster: delay, half-time to maximal and time to maximal responses were 15 times shorter with (-)Bay K 8644 than with Iso. It is concluded that frog ventricular myocytes respond slowly to a quick application of beta-adrenergic agonists.

Rate-limiting steps in the beta-adrenergic stimulation of cardiac calcium current

Fast-flow perfusion and flash photolysis of caged compounds were used to study the activation kinetics of L-type calcium current (ICa) in frog cardiac myocytes. Rapid exposure to isoproterenol (Iso) for 1 s or approximately 1 min produced similar kinetics of increase in ICa with an initial lag period of approximately 3 s, followed by a monophasic rise in current with a half-time of approximately 20 s. Epinephrine, as well as caged Iso, produced increases with similar kinetics. The fact that ICa increased significantly even after short Iso applications suggests that agonist binding to the receptor is rapid and that the increase in ICa is independent of free agonist. To dissect the kinetic contributions of various steps in the cAMP-phosphorylation cascade, the kinetics of the responses to caged cAMP and caged GTP gamma S and fast perfusion of forskolin, acetylcholine, and propranolol were compared. The response to caged cAMP exhibited no lag period, but otherwise increased at a rate similar to that produced by Iso and reached a peak at approximately 40 s after flash photolysis. This suggests that the lag period itself is due to a step before cAMP accumulation, but that activation of protein kinase and phosphorylation of the calcium channel are relatively slow. A lag period was also observed when ICa was stimulated by flash photolysis of caged GTP gamma S and when adenylyl cyclase was activated directly by rapid perfusion with forskolin. The lag period observed with forskolin may be due to slow binding of forskolin. The lag period was not due to the time required for cAMP to reach a threshold concentration, because a similar lag was observed in response to Iso in cells having ICa previously stimulated submaximally by internal perfusion with a low concentration of cAMP. These results suggest that the lag period can be attributed to a step associated with activation of adenylyl cyclase and cAMP accumulation.

Activation of L-type Ca2+ currents in cardiac myocytes by photoreleased GTP

L-type calcium currents (ICa) were recorded from isolated ventricular myocytes by using standard patch-clamp methods. In the absence of agonist, photorelease of GTP by flash photolysis of intracellularly applied caged-GTP rapidly increased the amplitude of ICa over a wide range of membrane potentials. Control experiments clearly demonstrated that this effect was not due to either the release of photolytic by-products or to the light flash itself. The timecourse for activation of ICa by photolysis of caged-GTP was markedly altered by intracellular application of either GDP beta S or GTP gamma S. Upon maximal stimulation of ICa by intracellular dialysis with cAMP, photoreleased GTP induced a small, rapid increase in ICa followed by a gradual inhibition. The presence of Rp-cAMPS intracellularly reduced both the magnitude of the response to photoreleased GTP and its time to peak. Similar effects were observed when protein kinase inhibitor dialysed the cell interior, suggesting that both cAMP-dependent and independent processes were involved in this effect. We conclude that rapid release of GTP within ventricular myocytes, in the absence of agonist, causes rapid activation of L-type Ca2+ current. Mechanisms underlying this effect include stimulation of adenylate cyclase, together with other, as yet uncharacterized, GTP-dependent pathways for increasing ICa in the heart.

Direct regulation of cardiac Ca2+ channels by G proteins: neither proven nor necessary?

The cardiac L-type Ca2+ channel has served as a model for ion channel regulation for over a decade. The Ca2+ current is increased by beta-adrenoceptor stimulation and this effect is inhibited by muscarinic acetylcholine receptor stimulation. It is well established that beta-adrenoceptor stimulation increases this current largely by cAMP-dependent phosphorylation but recently data have been presented that suggest that this channel may also be regulated directly by G proteins. This review by Criss Hartzell and Rodolphe Fischmeister evaluates evidence for this second regulatory pathway and concludes that, although G proteins affect cardiac Ca2+ channels in bilayers and excised patches, there is little evidence that this pathway is physiologically significant.

Long-chain fatty acids activate calcium channels in ventricular myocytes

Nonesterified fatty acids accumulate at sites of tissue injury and necrosis. In cardiac tissue the concentrations of oleic acid, arachidonic acid, leukotrienes, and other fatty acids increase greatly during ischemia due to receptor or nonreceptor-mediated activation of phospholipases and/or diminished reacylation. In ischemic myocardium, the time course of increase in fatty acids and tissue calcium closely parallels irreversible cardiac damage. We postulated that fatty acids released from membrane phospholipids may be involved in the increase of intracellular calcium. We report here that low concentrations (3-30 microM) of each long-chain unsaturated (oleic, linoleic, linolenic, and arachidonic) and saturated (palmitic, stearic, and arachidic) fatty acid tested induced multifold increases in voltage-dependent calcium currents (ICa) in cardiac myocytes. In contrast, neither short-chain fatty acids (less than 12 carbons) or fatty acid esters (oleic and palmitic methyl esters) had any effect on ICa, indicating that activation of calcium channels depended on chain length and required a free carboxyl group. Inhibition of protein kinases C and A, G proteins, eicosanoid production, or nonenzymatic oxidation did not block the fatty acid-induced increase in ICa. Thus, long-chain fatty acids appear to directly activate ICa, possibly by acting at some lipid sites near the channels or directly on the channel protein itself. We suggest that the combined effects of fatty acids released during ischemia on ICa may contribute to ischemia-induced pathogenic events on the heart that involve calcium, such as arrhythmias, conduction disturbances, and myocardial damage due to cytotoxic calcium overload.

Role of GTP-binding proteins in the regulation of mammalian cardiac chloride conductance

Beta-Adrenoceptor agonists activate a time- and voltage-independent Cl- conductance in mammalian cardiac myocytes. To characterize the cellular signaling pathways underlying its regulation, wide-tipped pipettes fitted with a pipette perfusion device were used to record whole-cell current and to introduce nucleotides to the interior of guinea pig ventricular myocytes. Replacement of pipette GTP with GDP beta S prevented activation of the Cl- conductance by Iso, suggesting a requirement for G protein turnover. With GTP in the pipette, the effect of Iso could be abolished by the beta-adrenoceptor antagonist propranolol, and mimicked by histamine or forskolin. These actions of Iso and forskolin are mediated exclusively via cAMP-dependent protein kinase (PKA), because (a) maximal activation of the Cl- conductance by forskolin or pipette cAMP occluded the effect of Iso, and (b) switching to pipette solution containing a synthetic peptide inhibitor (PKI) of PKA completely abolished the Cl- conductance activated by Iso and prevented the action of forskolin, but had no further effect. These results argue against basal activation of the Cl- conductance, and make it extremely unlikely that the stimulatory G protein, Gs, has any direct, phosphorylation-independent influence. The muscarinic receptor agonists acetylcholine (ACh) and carbachol diminished, in a reversible manner, Cl- conductance activated by Iso or forskolin, but not that elicited by cAMP. The muscarinic inhibition was abolished by replacing pipette GTP with GDP beta S, or by preincubating cells with pertussis toxin (PTX), and was therefore mediated by an inhibitory G protein, presumably Gi, influencing adenylyl cyclase activity. Nonhydrolyzable GTP analogues (GTP gamma S or GppNHp) applied via the pipette did not themselves activate Cl- conductance, but rendered Cl- current activation by brief exposures to Iso or histamine, but not to forskolin, irreversible. The Cl- conductance persistently activated by Iso was insensitive to propranolol or ACh, but could still be abolished by pipette application of PKI. The data indicate that stimulation of beta-adrenergic or histaminergic receptors in the presence of nonhydrolyzable GTP analogues causes persistent activation of Gs and uncouples it from the receptors. We conclude that autonomic regulation of cardiac Cl- conductance reflects accurately the underlying modulation of adenylyl cyclase activity and, hence, that this system is a suitable mammalian model for in situ studies of the interactions between adenylyl cyclase, Gs, Gi, and forskolin.

Histamine H3-receptor activation augments voltage-dependent Ca2+ current via GTP hydrolysis in rabbit saphenous artery

1. Actions of histamine on the voltage-dependent Ba2+(Ca2+) currents (IBa, ICa) were investigated using the whole-cell patch-clamp technique on dispersed smooth muscle cells from the rabbit saphenous artery. 2. Histamine (half-maximal dose, EC50 = 530 nM) augmented the IBa evoked by a brief depolarizing pulse (100 ms duration; to +10 mV from a holding potential of -80 mV) in a concentration-dependent manner. The maximum augmentation was obtained with 30 microM-histamine (1.29 times control). This augmentation of IBa was inhibited by the H3-antagonist, thioperamide (Ki = 30 nM, slope of the Schild plot = 1.0), but not by H1- or H2-antagonists (mepyramine or diphenhydramine, or cimetidine, respectively). 3. An H3-agonist, R alpha-methylhistamine (EC50 = 93 nM), also augmented IBa in a concentration-dependent manner at a holding potential of -80 mV and the maximum augmentation (1.25 times control) was obtained with 10 microM. This augmentation was also inhibited by thioperamide, but not by the above H1- and H2- antagonists. 4. Intracellularly applied 500 microM-guanosine 5'-triphosphate (GTP) enhanced, but 1 mM-guanosine 5'-O-(2-thiodiphosphate) (GDP beta S) abolished, the histamine-induced augmentation of IBa. When one of the non-hydrolysable GTP analogues, guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S; greater than 5 microM), guanylyl-imidodiphosphate (GMP-PNP; 200 microM) or guanylyl (beta, gamma-methylene)-diphosphonate (GMP-PCP; 1 mM) was intracellularly applied, the IBa amplitude evoked without the application of histamine was not affected, but the excitatory effect of histamine on IBa was reversed to an inhibition. Pre-treatment with pertussis toxin (PTX: 300 ng/ml and 3 micrograms/ml) did not modify the histamine-induced responses in the absence or presence of GTP gamma S. 5. 4 beta-Phorbol 12,13-dibutylate (PDBu) increased the amplitude of IBa. However, this action of PDBu was not enhanced by the application of GTP (500 microM) in the pipette, but additional application of histamine further increased the amplitude of IBa. Pre-treatment with a potent non-selective protein kinase inhibitor, 1-(5-isoquinolinesulphonyl)-2-methylpiperazine dihydrochloride (H-7; 100 microM), did not modify the histamine-induced current augmentation or inhibition observed in the presence or absence of intracellular GTP gamma S.(ABSTRACT TRUNCATED AT 400 WORDS)

Role of G proteins in stimulation of Na-H exchange by cell shrinkage

Many cells respond to shrinkage by stimulating specific ion transport processes (e.g., Na-H exchange). However, it is not known how the cell senses this volume change, nor how this signal is transduced to an ion transporter. We have studied the activation of Na-H exchange in internally dialyzed barnacle muscle fibers, measuring intracellular pH (pHi) with glass microelectrodes. When cells are dialyzed to a pHi of approximately 7.2, Na-H exchange is active only in shrunken cells. We found that the shrinkage-induced stimulation of Na-H exchange, elicited by increasing medium osmolality from 975 to 1,600 mosmol/kgH2O, is inhibited approximately 72% by including in the dialysis fluid 1 mM guanosine 5'-O-(2-thiodiphosphate). The latter is an antagonist of G protein activation. Even in unshrunken cells, Na-H exchange is activated by dialyzing the cell with 1 mM guanosine 5'-O-(3-thiotriphosphate), which causes the prolonged activation of G proteins. Activation of Na-H exchange is also elicited in unshrunken cells by injecting cholera toxin, which activates certain G proteins. Neither exposing cells to 100 nM phorbol 12-myristate 13-acetate nor dialyzing them with a solution containing 20 microM adenosine 3',5'-cyclic monophosphate (cAMP) (or 50 microM dibutyryl cAMP) plus 0.5 mM 3-isobutyl-1-methylxanthine substantially stimulates the exchanger. Thus our data suggest that a G protein plays a key role in the transduction of the shrinkage signal to the Na-H exchanger via a pathway that involves neither protein kinase C nor cAMP.

Excitation-contraction coupling in skeletal muscle fibres of rat and toad in the presence of GTP gamma S

1. Rapid force responses were elicited in single mechanically skinned fibres from extensor digitorum longus (EDL) muscle of the rat when the fibres were depolarized by substituting K+ in the bathing solution with Na+. The properties of these depolarization-induced responses, the responses to lowered [Mg2+], and the characteristics of the slow prolonged response ('second component') produced in 'loaded' fibres by choline chloride (ChCl) substitution, were virtually identical to those observed previously in skinned fibres from toad muscle. 2. At physiological levels of [Mg2+] (1 mM) and Ca2+ loading, application of 50 microM- to 1 mM-GTP gamma S (guanosine-5'-O-(3-thiotriphosphate), a non-hydrolysable analogue of GTP) did not produce a response in any mammalian or amphibian fibre, even though the depolarization-induced coupling was totally functional. Furthermore, the presence of GTP gamma S had no apparent effect on the size, the threshold or the maximum number of responses which could be elicited by depolarization. 3. GTP gamma S did not elicit any response when excitation-contraction coupling was abolished by prolonged depolarization or by chemically skinning the fibre with saponin or by 24 h exposure to low [Ca2+] (5 mM-EGTA). 4. GDP beta S (guanosine-5'-O-(2-thiodiphosphate), 250 microM or 1 mM) neither evoked a response nor affected the responses to depolarization or caffeine. 5. When the [Mg2+] was lowered to 0.2 mM and the fibres were heavily loaded with Ca2+, addition of GTP gamma S (250 microM or 1 mM) induced a small response in about 50% of fibres, but depolarization-induced responses were not affected in any fibres. 6. Asymmetric charge movement recorded in EDL fibres with the vaseline-gap voltage clamp was not affected by the application of 1 mM-GTP gamma S to the cut ends of the fibres for up to 1 h. 7. These data imply that GTP-binding proteins (G-proteins) are not involved in coupling the voltage sensors to Ca2+ release in skeletal muscle. Furthermore, there was no evidence that G-proteins play any role in modulating the voltage sensors, though this possibility could not be totally excluded.

Role of the GTP-binding protein Gs in the beta-adrenergic modulation of cardiac Ca channels

In the heart, the guanosine 5'-triphosphate (GTP)-binding protein Gs is activated by hormone binding to beta-adrenergic receptors and stimulates the intracellular cyclic adenosine 3',5'-monophosphate (cAMP) pathway that leads to phosphorylation of L-type Ca channels by the cAMP-dependent protein kinase A. Additionally, Gs can modulate cardiac Ca channels directly in cell-free systems. In order to examine the question of whether these pathways could be separated functionally and whether they act independently or synergistically on L-type Ca channels in intact cells, the whole-cell Ca current (ICa) and the respective current density were measured in guinea-pig ventricular myocytes at 0 mV. The following results were obtained. First, typically, the ICa density increased from 12 to 40 microA/cm2 following application of 1 microM isoproterenol (ISP) to myocytes bathed in solutions containing 1.8 mM CaCl2. However, 1 microM ISP enhanced ICa only from 9 to 17 microA/cm2 after inhibition of the protein kinase A by dialysis of 0.5 mM Rp-cAMPs (the Rp-isomer of adenosine 3',5'-monophosphorothioate) in the presence of 0.5 mM GTP. Withdrawal of GTP from the dialysate attenuated the effects of ISP on ICa. Thus, Rp-cAMPS unmasks a GTP-dependent component of the beta-adrenergic stimulation of ICa, which probably reflects the direct stimulation of Ca channels by Gs under block of cAMP-dependent phosphorylation. Second, in cells under dialysis with 100 or 200 microM cAMP, bath application of 20-40 microM 3-isobutyl-1-methylxanthine (IBMX) enhanced the ICa density to about 41 microA/cm2 indicating saturation of the cAMP pathway. Under this condition, 1 microM ISP was without significant effect on ICa. This result may suggest that direct Gs stimulation is rather ineffective on Ca channels after maximal cAMP-dependent phosphorylation. Alternatively, maximal stimulation of the cAMP pathway may also interfere with the activation of the Gs pathway in intact myocytes. Third, simultaneous application of 1 microM ISP and 40 microM IBMX enhanced ICa up to densities of around 75 microA/cm2 during cell dialysis with 100 microM cAMP, an effect much stronger than that exerted by IBMX alone under similar conditions. Since it seems likely that Gs is activated more quickly, than the cAMP pathway during application of the ISP/IBMX mixture, the latter result suggests that a direct effect of Gs may act to prime L-type Ca channels for cAMP-dependent phosphorylation during beta-adrenergic stimulation of cardiac myocytes.

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.

Effects of neuropeptide Y on cell length and membrane currents in isolated guinea pig ventricular myocytes

Direct effects of neuropeptide Y were studied in left ventricular myocytes isolated from guinea pigs. Contraction was measured as the change in unloaded cell length using a photodiode array. Action potentials were elicited at 1 Hz in current-clamp mode, and membrane currents were measured using a switch-clamp amplifier with 2 M-KCl microelectrodes. At concentrations of 10(-6) M and above, neuropeptide Y reduced contraction in a concentration-dependent fashion. The reduction in contraction by the peptide was proportionately greater in the presence of isoproterenol, and the increase in contraction caused by isoproterenol was completely inhibited by 10(-6) M neuropeptide Y. In response to neuropeptide Y, action potential duration was shortened, and the time course of the shortening was similar to that of the reduction in contraction. Under voltage clamp, 1 x 10(-5) M neuropeptide Y reduced peak L-type calcium current by 32% and shifted the myocyte current-voltage relation during a slow ramp in a manner that suggested a reduction in the background rectifier K+ current. The effects of the peptide on membrane currents were greatly attenuated by preincubation of the cells with pertussis toxin (100 ng/ml). We conclude that neuropeptide Y reduces developed shortening, action potential duration, L-type calcium current, and background rectifier current in single guinea pig ventricular myocytes and that these effects are mediated, at least in part, via membrane G proteins.

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.

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.

G proteins and calcium channels in myocardium

Calcium mobilization has been demonstrated to possess functional importance in myocardial excitation-contraction as well as cellular metabolism. So far, much progress has been made to explore the possibility of involvement of guanine nucleotide binding regulatory protein in the opening and closing of calcium channels as well as intracellular second messenger (cyclic adenosine monophosphate and inositol triphosphate) -mediated calcium mobilization, although such work is still in its preliminary stage, the results have proved highly interesting and significant.