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

8-bromo-cGMP reduces the myofilament response to Ca2+ in intact cardiac myocytes

The role of cGMP in myocardial contraction is not established. Recent reports suggest that nitric oxide, released by endothelial cells or within myocytes, modifies myocardial contraction by raising cGMP. We studied the effects of 8-bromo-cGMP (8bcGMP, 50 mumol/L) on contraction (cell shortening) and simultaneous intracellular Ca2+ transients (indo 1 fluorescence ratio) in intact adult rat ventricular myocytes (0.5 Hz and 25 degrees C) 8bcGMP reduced myocyte twitch amplitude and time to peak shortening (-19.6 +/- 4.2% and -17.6 +/- 1.3%, respectively) and increased steady-state diastolic cell length (+0.6 +/- 0.1 microns, mean +/- SEM, n = 8; all P < .05) but had no effect on shortening velocity, systolic or diastolic fluorescence ratio, or time to peak fluorescence ratio (all P = NS). In 7 of 13 myocytes, this negative inotropic effect was preceded by a transient positive inotropic effect, with small increases in twitch amplitude, shortening velocity, and cytosolic Ca2+ transient. Analysis of 8bcGMP effects on both the dynamic and steady-state relation between cell shortening and intracellular Ca2+ (during twitch contraction and tetanic contraction, respectively) indicated reduction in the myofilament response to Ca2+ in all cases. These 8bcGMP effects were inhibited by KT5823 (1 mumol/L), an inhibitor of cGMP-dependent protein kinase, or by the presence of isoproterenol (3 nmol/L). 8bcGMP had no effect on cytosolic pH in cells (n = 4) loaded with the fluorescent probe carboxyseminaphthorhodafluor-1. These data indicate that cGMP may modulate myocardial relaxation and diastolic tone by reducing the relative myofilament response to Ca2+, probably via cGMP-dependent protein kinase.

Guanylate-cyclase-mediated inhibition of cardiac ICa by carbachol and sodium nitroprusside

We studied the role of cyclic guanosine monophosphate (cGMP) as a mediator of the reduction of L-type calcium current (ICa) induced by muscarinic receptor stimulation and by nitric oxide in isolated guinea-pig ventricular cells using the whole-cell patch-clamp technique. Our results show that when the level of cyclic adenosine monophosphate was increased by the phosphodiesterase inhibitor isobutylmethylxanthine (IBMX), stimulation of a pertussis-toxin (PTX)-sensitive muscarinic receptor by carbachol (1 microM) reduced the calcium current increase from 80.6 +/- 23.5% to 19.8 +/- 9.6% over the control and this effect was prevented by methylene blue (10 microM), an inhibitor of the soluble guanylate cyclase. Pipette solution containing 10 microM cGMP reduced the enhancement of ICa by IBMX from 121.9 +/- 11.6% to 14.2 +/- 5.4% above the control. Sodium nitroprusside (10 microM), a spontaneous donor of nitric oxide, and consequently a stimulator of soluble guanylate cyclase, also reduced IBMX-stimulated ICa from 115.2 +/- 13.2% to 32.2 +/- 6.9% above control and the sodium nitroprusside effect was also suppressed by methylene blue. The latter two reagents were ineffective on basal ICa.

Cyclic GMP regulation of calcium slow channels in cardiac muscle and vascular smooth muscle cells

The effects of cAMP and cGMP on the slow Ca2+ channels in cardiac muscle, VSM, and skeletal muscle fibers are summarized in Table V. As shown, in cardiac muscle, cAMP stimulates and cGMP inhibits. In VSM, both cAMP and cGMP inhibit. In skeletal muscle, both cAMP and cGMP stimulate.

Regulation of calcium slow channels of heart by cyclic nucleotides and effects of ischemia

The slow Ca2+ channels (L-type) of the heart are stimulated by cAMP. Elevation of cAMP produces a very rapid increase in the number of slow channels available for voltage activation during excitation. The probability of a Ca2+ channel opening and the mean open time of the channel are increased. Therefore, any agent that increases the cAMP level of the myocardial cell will tend to potentiate ICa, Ca2+ influx, and contraction. The action of cAMP is mediated by PK-A and phosphorylation of the slow Ca2+ channel protein or an associated regulatory protein (stimulatory type). The myocardial slow Ca2+ channels are also regulated by cGMP, in a manner that is opposite or antagonistic of that of cAMP. This has been demonstrated at both the macroscopic level (whole-cell voltage clamp) and the single-channel level. The effect of cGMP is mediated by PK-G and phosphorylation of a protein, for example, a regulatory protein (inhibitory type) associated with the Ca2+ channel. It has been demonstrated that introduction of PK-G intracellularly causes a relatively rapid inhibition of ICa(L) in both chick and rat heart cells. In addition, cGMP/PK-G act to stimulate a phosphatase that dephosphorylates the Ca2+ channel. In addition to the slower, indirect pathway--exerted via cAMP/PK-A--there is a faster, more direct pathway for ICa(L) stimulation by the beta-adrenergic receptor. The latter pathway involves direct modulation of the channel activity by the alpha subunit (alpha S*) of the GS protein. PK-C and calmodulin-PK also may play roles in the regulation of the myocardial slow Ca2+ channels, possibly mediated by phosphorylation of some regulatory type of protein. Both protein kinases stimulate the activity of the slow Ca2+ channels. Thus, it appears that the slow Ca2+ channel is a complex structure, including perhaps several associated regulatory proteins, which can be regulated by a number of factors intrinsic and extrinsic to the cell (Fig. 9). The cyclic nucleotides also have effects on the slow Ca2+ channels in cells other than cardiac muscle, including neurons, smooth muscle, and skeletal muscle fibers (Tables III and IV). In cardiac muscle, the two cyclic nucleotides have opposing effects, cAMP stimulating and cGMP inhibiting. In some smooth muscles (e.g., vascular), both cyclic nucleotides act in the same direction, namely, both inhibit ICa(L). In skeletal muscle, both cAMP and cGMP act in the same direction on ICa(L), but to stimulate.(ABSTRACT TRUNCATED AT 400 WORDS)

Synergistic inhibitory effects of atriopeptin II and isoproterenol on contraction of rat aortic smooth muscle: roles of cGMP and cAMP

Atriopeptin II and isoproterenol acted synergistically to inhibit the phenylephrine-induced contraction of aortic smooth muscle from Wistar-Kyoto (WKY) rats. Thus, a weakly inhibitory concentration of atriopeptin II (10 nM) caused a 5-fold decrease in the IC50 of isoproterenol from 169 nM to 32 nM, whereas a low concentration of isoproterenol (100 nM) increased the maximum inhibition attributable to atriopeptin II from 43% to 74%. Atriopeptin II (10 nM) increased the cGMP found in aortic smooth muscle and approximately doubled the accumulation of cAMP caused by isoproterenol. The results suggest that cGMP, formed by the action of atriopeptin II on receptor guanylyl cyclase (GC-A), may inhibit aortic cyclic nucleotide phosphodiesterase type III (PDE III) and that an increased accumulation of cAMP then mediates the observed synergism.

The enhancement of catecholamine-induced Cl- current by cyclic GMP revealed using photolabile caged compounds in guinea-pig ventricular cells

Whole cell currents were recorded in single myocytes dissociated from guinea-pig ventricles, and caged compounds were loaded intracellularly through the patch electrodes. Flash photolysis of caged cyclic GMP (cGMP) increased the amplitudes of both catecholamine-induced Cl- (ICl) and Ca2+ currents (ICa) which were pre-activated by submaximum doses of isoprenaline. Transient activation of ICl by photo-release of cyclic AMP (cAMP) showed a half decay time (t1/2) of 16.7 +/- 1.4 sec (mean +/- S.E.M., n = 14). This decay was markedly delayed by inhibiting phosphodiesterases using 3-isobutyl-1-methyl-xanthine (IBMX). The intracellular application of cGMP (10-50 microM) also prolonged the decay of the ICl response to caged cAMP (t1/2 = 38.0 +/- 7.1 sec, n = 12). These findings strongly support the hypothesis that cGMP facilitates the beta-adrenergic response of ionic currents through the inhibition of phosphodiesterase in mammalian cardiac myocytes.

Acetylcholine reversal of isoproterenol-stimulated sodium currents in rabbit ventricular myocytes

We have recently shown that beta-adrenergic agonists enhance the cardiac sodium current (INa) in rabbits through dual G-protein regulatory pathways. To determine if muscarinic cholinergic receptor stimulation can also modulate INa, we studied the effects of acetylcholine (ACh) and carbachol on INa in enzymatically dispersed rabbit ventricular myocytes. Whole-cell patch-clamp experiments done at room temperature using 20 mM [Na+]o showed that 100 nM isoproterenol increased INa and accelerated current decay as previously described. ACh (1 microM) or carbachol (1 microM) significantly reversed the stimulatory isoproterenol effects at test potentials throughout the INa activation range and at holding potentials negative to -80 mV. This effect was completely inhibited by atropine (1 microM) and was confirmed by studying single-channel INa from cell-attached patches. When INa was stimulated by forskolin (1 microM), carbachol (1 microM) significantly reversed the effect. The muscarinic-mediated inhibition of INa was inhibited by pertussis toxin (0.1 or 1.0 microgram/ml) incubation (12-15 hours), suggesting that the effect was inhibitory G-protein dependent. Further investigation of the ACh inhibitory mechanism revealed that ACh alone had no effect on INa and that when cells were dialyzed with cAMP (5 microM), ACh failed to inhibit INa. Furthermore, cGMP failed to inhibit the effect of isoproterenol on INa. These data suggest that ACh acts at or proximal to adenylate cyclase stimulation. Thus, rabbit cardiac Na+ channels are regulated by muscarinic agonists in a fashion similar to cardiac Ca2+ channels.

Modulation of L-type Ca channel activity by P2-purinergic agonist in cardiac cells

The mechanism of enhancement of the L-type Ca current by a P2-purinergic agonist adenosine-5'-O-(3-thiotriphosphate) (ATP gamma S) was studied by recording single channel activity from cell-attached patches on rat isolated ventricular cells using patch pipettes containing 110 mM Ba2+. The application of ATP gamma S to the patch membrane through the pipette solution did not affect single channel activity. The addition of ATP gamma S to the bath containing a depolarizing solution was ineffective due to the voltage dependence of the purinergic stimulation. Bath application of ATP gamma S (100 microM) to control 4-(2-hydroxyethyl)-1-piperazine-ethanesulphonic acid (HEPES) solution increased the amplitude of ensemble average currents both by decreasing the probability of a blank sweep occurring and by increasing the number of openings per non-blank sweep. The single channel conductance (17 pS) was not changed by ATP gamma S. Both activation and inactivation curves were shifted towards hyperpolarized potentials by about 10 mV under P2-purinergic stimulation. Since ATP gamma S increased channel activity when applied via the bath, it must be supposed that a diffusible messenger is involved.

Effects of atrionatriuretic factor on Ca2+ current and Cai-independent transient outward K+ current in human atrial cells

The effect of 10 nM atrial natriuretic peptide (ANF) on macroscopic L-type calcium current, ICa, and calcium-independent outward potassium current, Ilo, were studied in myocytes isolated from human atrial trabeculae using the whole-cell-recording patch-clamp technique. When cells were dialysed with pipette media containing 0.2 mM GTP, ANF reduced ICa by 37.81% +/- 5.4% at +20 mV and Ilo by 21.72% +/- 3.68% at +60 mV in a reversible manner. When ICa was increased by beta-adrenoreceptor stimulation (0.1 microM isoproterenol) or by the phosphodiesterase inhibitor isobutylmethylxanthine (10 microM) ANF reduced ICa by 24.99 +/- 3.4% and by 39.9 +/- 6.3% respectively. In cells dialysed with GTP-free pipette media, ANF increased ICa markedly (39.8% +/- 7%) and reversibly, whereas it still depressed Ilo (18.92% +/- 2%). Addition of 0.2 mM GTP[gamma S] to the pipette solution in the absence of GTP increased ICa, decreased Ilo and suppressed the effect of ANF on both ICa and Ilo. It is suggested that activation of the ANF receptor in human atrial cells reduces ICa via guanylate-cyclase-dependent cGMP production, increases ICa via Gs protein activation and decreases Ilo via Gi protein activation.

Synergistic action of cyclic GMP on catecholamine-induced chloride current in guinea-pig ventricular cells

1. Effects of cyclic GMP on the catecholamine-induced chloride current (ICl) were studied using the whole-cell patch-clamp technique combined with internal perfusion in single ventricular myocytes dispersed from guinea-pig heart. 2. When ICl was activated by submaximal doses of isoprenaline (0.01-0.1 microM), adrenaline (0.5-1 microM) and histamine (0.2-0.5 microM), intracellular dialysis with cyclic GMP (10-100 microM) induced an extra increase of ICl. No further increase of ICl was induced by cyclic GMP when ICl was maximally activated. In the absence of agonists, cyclic GMP failed to induce ICl. 3. The enhancement by cyclic GMP was also observed when ICl was activated by external application of 0.2-1.0 microM-forskolin or by internal dialysis with a pipette solution containing 50-200 microM-cyclic AMP. 4. In contrast to cyclic GMP, 10-1000 microM-dibutyryl cyclic GMP and 8-bromo-cyclic GMP were ineffective in modifying ICl. 5. Milrinone (1-10 microM), a specific inhibitor of a kind of phosphodiesterase which is inhibited by cyclic GMP, also enhanced ICl activated by submaximal doses of isoprenaline. Milrinone itself did not activate ICl. 6. When ICl was enhanced by 5 microM-milrinone, an additional application of cyclic GMP failed to increase ICl. In the presence of cyclic GMP, milrinone failed to enhance ICl. 7. The above findings on ICl are analogous to the enhancement by cyclic GMP of the beta-adrenergic stimulation of the Ca2+ current reported in the same preparation, and support the hypothesis that in mammalian cardiac cells cyclic GMP potentiates elevation of cyclic AMP induced by beta-adrenergic agents, and thereby increases the amplitudes of ionic currents.

Signal transduction by cGMP in heart

Early studies in whole heart indicated that cGMP antagonized the positive inotropic effects of catecholamines and cAMP. However, the regulation of cGMP levels by a variety of agents was not always consistent with their effects on contractility. It is now clear that at least two major cell types in whole heart, cardiac myocytes and vascular smooth muscle cells, differ markedly in their mechanisms of cGMP regulation and response to cGMP. Furthermore, experiments on isolated cardiac myocytes indicate that the mechanism of cGMP action even in this single cell type can be multifaceted. Cyclic GMP inhibits the L-type calcium channel current (ICa), which is the major source of Ca++ entry into heart cells, and which plays a predominant role in the initiation and regulation of cardiac electrical and contractile activities. Patch-clamp measurements of ICa indicate that in isolated frog myocytes cGMP inhibits ICa by stimulation of cAMP phosphodiesterase (cGS-PDE), whereas in purified rat ventricular myocytes, cGMP predominantly inhibits ICa via a mechanism involving cGMP-dependent protein kinase (cGMP-PK). Under certain conditions, cGMP can also inhibit a cGMP-inhibited cAMP phosphodiesterase (cGI-PDE) and thereby produce a stimulatory effect on ICa. Biochemical characterization of the endogenous PDEs and cGMP-PK in purified cardiac myocytes provided further evidence in support of these mechanisms of cGMP action on ICa.