Nitric oxide regulates the calcium current in isolated human atrial myocytes

Relationship between decreased function and O2 consumption caused by cyclic GMP in cardiac myocytes and L-type calcium channels

We tested the hypothesis that part of the decreased function and metabolism caused by cyclic guanosine monophosphate (GMP) in beating cardiac myocytes is related to inhibition of L-type calcium channels. The steady state oxygen consumption (VO2) of a suspension of ventricular myocytes isolated from hearts of New Zealand white rabbits was measured using oxygen electrodes. Cellular cyclic GMP levels were determined by radioimmunoassay. Cell shortening was measured with a video edge detector. The VO2 was obtained after: (1) adding sodium nitroprusside (NP 10(-8),(-6),(-4) M), (2) pretreatment by BAY K8644 10(-5) M (BAY, L-type calcium channel activator), nifedipine 10(-4) M (NF, L-type calcium channel blocker) or forskolin 10(-7) M (FK, adenylate cyclase activator), then adding NP 10(-8),(-6),(-4) M, (3) pretreatment with both FK 10(-7) M and NF 10(-4) M and subsequently adding NP 10(-8),(-6),(-4) M. NP 10(-4) M decreased VO2 from 707 +/- 34 to 410 +/- 13 (nl O2/min per 10(5) myocytes), decreased the percentage of shortening (Pcs) from 5.7 +/- 0.6 to 3.7 +/- 0.5 and the rate of shortening (Rs) from 65.5 +/- 4.5 (microns/s) to 46.2 +/- 5.5. NP 10(-4) M also increased cyclic GMP from 264 +/- 70 (fmol/10(5) myocytes) to 760 +/- 283. Both BAY and FK increased VO2, Pcs and Rs without changing cyclic GMP. NF decreased Pcs, Rs and VO2. Similar metabolic and functional effects of NP were observed with pretreatment with these agents separately, compared to NP alone, and the elevation of cyclic GMP level was not different from the control group. With FK alone, NP 10(-4) M decreased VO2 by 51%, Pcs by 44% and Rs by 39%. In the presence of both FK and NF, the negative effects of NP were diminished significantly. NP 10(-4) M decreased VO2 by 37%, Pcs by 25% and Rs 20%. Thus, in beating cardiac myocytes, the negative metabolic and functional effects of cyclic GMP were related to inhibition on L-type calcium channels only when adenylate cyclase was stimulated.

Negative metabolic effects of cyclic GMP in quiescent cardiomyocytes are not related to L-type calcium channel activity

We tested the hypothesis that the negative metabolic effects of elevating cyclic GMP act through inhibition of L-type calcium channels in quiescent cardiac myocytes. The steady state O2 consumption (VO2) of ventricular myocytes, isolated from hearts of New Zealand white rabbits, was measured in a glass chamber using Clark-type oxygen electrodes. The cellular cyclic GMP levels were determined by radioimmunoassay at baseline with either 0.5 mM or 2.0 mM of Ca2+, sodium nitroprusside at increasing concentration (10(-8),(-6),(-4) M) with and without pretreatment by BAY K8644 10(-5) M (L-type Ca2+ channel activator) in 0.5 mM Ca2+, or nitroprusside with and without pretreatment with nifedipine 10(-4) M (L-type Ca2+ channel blocker) in 2.0 mM Ca2+. In the 0.5 mM Ca2+ medium, basal VO2 was 459 +/- 104 (nl O2/min per 10(5) myocytes) with a corresponding cyclic GMP level of 112 +/- 23 (fmol/10(5) myocytes). With nitroprusside 10(-4) M, VO2 was decreased to 285 +/- 39 and cyclic GMP level was significantly elevated to 425 +/- 128. In the same medium, VO2 was slightly increased by BAY K8644 10(-5) M while the cyclic GMP level did not change. With BAY K8644 10(-5) M, nitroprusside 10(-4) M decreased VO2 and increased cyclic GMP to a level which was similar to cells treated with nitroprusside alone. In the 2.0 mM Ca2+ medium, the basal VO2 and cyclic GMP were 518 +/- 121 and 137 +/- 24. In the presence of nitroprusside 10(-4) M, VO2 was decreased to 295 +/- 49 and cyclic GMP was increased to 454 +/- 116. In the same medium, nifedipine 10(-4) M significantly decreased VO2, while the cyclic GMP level was comparable to the baseline. After nifedipine 10(-4) M, nitroprusside 10(-4) M decreased VO2 and increased cyclic GMP to levels which were similar to control. Therefore, in quiescent cardiac myocytes, the negative metabolic effects associated with cyclic GMP were not primarily mediated through inhibition of L-type Ca2+ channels.

Aspects of novel sites of regulation of the insulin stimulus-secretion coupling in normal and diabetic pancreatic islets

Noninsulin-dependent diabetes mellitus (NIDDM), a major health care problem in the Western world, is a disease typified by a relative deficiency of insulin, leading to vast derangements in glucose and lipid homeostasis with disastrous vascular complications. Despite immense research efforts aimed at a clear understanding of the etiology of this complex disease, the molecular mechanisms causing the disorder still remain elusive. This article reviews extant data from recent publications implicating novel signal transduction pathways as important regulators of the insulin stimulus-secretion coupling in the pancreatic beta-cell. The significance of nitric oxide and serine/threonine protein phosphatases, and their inactivation by insulin secretagogues, glucose metabolites, ATP, GTP, glutamate, and inositol hexaphosphate in this arena is scrutinized. Additionally, also presented is the growing concept that an important signal for insulin secretion may reside in the inextricable interplay between glucose and lipid metabolism, specifically the generation of malonyl-CoA, which inhibits carnitine palmitoyltransferase 1 with the attendant accumulation of long-chain acyl CoA esters. Moreover, attention is directed towards novel intracellular actions of hypoglycemic sulfonylureas in the beta-cell. Finally, the importance of "lipotoxicity" and aberrations in glucose uptake and metabolism in beta-cell dysfunction is given consideration. Future research efforts should aim at further characterization of effects of second messengers on protein phosphorylation elements in beta-cells. Additionally, long-term regulation by glucose and the diabetic state (e.g., fatty acids and ketones) on beta-cell protein phosphatases, pyruvate dehydrogenase, and carnitine palmitoyltransferase 1 needs to be explored in greater depth. Clearly, the detrimental impact of diabetic hyperlipidemia on beta-cell function has been a relatively neglected area, but futu re pharmacological approaches directed at preventing lipotoxicity may prove beneficial in the treatment of diabetes.

Genistein elicits biphasic effects on L-type Ca2+ current in feline atrial myocytes

A perforated patch recording method was used to determine the effects of genistein (Gen), a protein tyrosine kinase (PTK) inhibitor, on basal L-type Ca2+ current (ICa,L) in feline atrial myocytes. Gen (50 microM) elicited biphasic changes in ICa,L: an initial inhibition (-55 +/- 4%; phase 1) followed by a secondary stimulation (34 +/- 9%; phase 2) of ICa,L. Withdrawal of Gen elicited a further potentiation of ICa,L (152 +/- 19%; phase 3) above control (n = 46). In general, phase 1 inhibition and phase 3 potentiation varied directly with Gen concentration, and phase 2 stimulation exhibited biphasic concentration-dependent changes compared with control. When cells were dialyzed using a ruptured patch recording method, Gen elicited only inhibition of ICa,L; phases 2 and 3 were abolished. Vanadate (1 mM), an inhibitor of protein tyrosine phosphatase, abolished both Gen-induced inhibition and stimulation of ICa,L. Daidzein (50 microM), a weakly active analog of Gen, exerted no significant effects on ICa,L, and withdrawal of daidzein failed to potentiate ICa,L. In a few cells, Gen elicited a prominent vanadate-sensitive stimulation of ICa,L in the absence of any significant inhibition of ICa,L. Gen-induced changes in ICa,L were unaffected by either 100 microM 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA)-acetoxymethyl ester (AM) or 1 microM ryanodine, agents that alter intracellular Ca2+; 4 microM H-89 or 50 microM Rp diastereomer of adenosine 3',5'-monophosphothioate (RP-cAMPS), inhibitors of protein kinase A (PKA); 0.1 microM calphostin C or 2 microM chelerythrine, inhibitors of protein kinase C (PKC); or 100 microM NG-monomethyl-L-arginine (L-NMMA), an inhibitor of nitric oxide (NO) synthase. We conclude that in feline atrial myocytes, Gen acts via membrane-bound PTKs to inhibit ICa,L and via cytosolic PTKs to stimulate ICa,L. Gen-induced changes in ICa,L are not related to changes in intracellular Ca2+ or to secondary interactions with either PKA, PKC, or NO signaling pathways. These results indicate that in atrial myocytes ICa,L is regulated by two independent and competing PTK signaling mechanisms.

Expression and characterization of deletion recombinants of two cGMP-inhibited cyclic nucleotide phosphodiesterases (PDE-3)

cDNAs encoding two PDE-3 or cyclic GMP-inhibited (cGI) cyclic nucleotide phosphodiesterase (PDE) isoforms, RPDE-3B (RcGIP1) and HPDE-3A (HcGIP2), were cloned from rat (R) adipose tissue and human (H) heart cDNA libraries. Deletion and N- and C-terminal truncation mutants were expressed in Escherichia coli in order to define their catalytic core. Active mutants of both RPDE-3B and HPDE-3A included the domain conserved among all PDEs plus additional upstream and downstream sequences. An RPDE-3B mutant consisting of the conserved domain alone and one from which the RPDE-3B 44-amino acid insertion was deleted exhibited little or no activity. All active recombinants exhibited a high affinity (< 1 microM) for cyclic AMP (cAMP) and cyclic GMP (cGMP), were inhibited by cAMP, cGMP, and cilostamide, but not by rolipram, and were photolabeled with [32P]-cGMP. The IC50 values for cGMP inhibition of cAMP hydrolysis were lower for HPDE-3A than for RPDE-3B recombinants. The deduced amino acid sequences of HPDE-3A and RPDE-3B catalytic domains are very similar except for the 44-amino acid insertion not found in other PDEs. It is possible that this insertion may not only distinguish PDE-3 catalytic domains from other PDEs and identify catalytic domains of PDE-3 subfamilies or conserved members of the PDE-3 gene family, but may also be involved in the regulation of sensitivity of PDE-3s to cGMP.

The effects of intrinsic nitric oxide on cardiac neural regulation in cats

In this study, we aimed to elucidate the effects of intrinsic nitric oxide (NO) on cardiac neural regulation. Twenty-two cats were anesthetized with 1.5% isoflurane and allocated to Group I (intact; n = 7), Group D (denervated baroreceptors and vagi; n = 8), or Group B (autonomic blockade with i.v. hexamethonium, propranolol, and atropine; n = 7). Cardiac sympathetic nerve activity (CSNA), mean arterial pressure (MAP), sinus heart rate (HR), and A-H and H-V intervals during pacing (150 bpm) were measured before and after i.v. administration of a NO synthase inhibitor, NG-nitro-L-arginine (L-NNA, 30 mg/kg) and after reversal with an excessive dose of L-arginine (300 mg/kg), before and during intermittent electrical stimulation of the posterior hypothalamus. L-NNA significantly increased MAP in Groups I and B, but not in Group D. L-NNA significantly decreased HR and lengthened A-H in Group I, but not in other groups. L-arginine further decreased HR and lengthened A-H unexpectedly. The reasons for these findings could not be determined in this study. L-NNA did not change CSNA. Hypothalamic stimulation did not potentiate L-NNA-induced changes in CSNA, hemodynamic variables, and atrioventricular conduction. In conclusion, intrinsic NO may modulate atrioventricular conduction and sinus rate through a vagal cholinergic, rather than a nonautonomic mechanism.

IMPLICATIONS

Elucidating the roles of intrinsic nitric oxide (NO) on cardiac neural regulation is important. In intact, vagotomized, and baroreceptor-denervated or pharmacologically autonomic blockaded cats, an NO synthesis inhibitor was administered, and atrioventricular conduction and cardiac sympathetic neural discharge were measured. The results suggest a vagal cholinergic mechanism of intrinsic NO.

Exogenous nitric oxide reduces oxygen consumption of isolated ventricular myocytes less than other forms of guanylate cyclase stimulation

We tested the hypothesis that increasing cyclic GMP with nitric oxide (NO) would reduce cardiac myocyte metabolism less than other forms of guanylate cyclase stimulation. The steady state O2 consumption (VO2) of a suspension of ventricular myocytes in 2.0 mM Ca2+ isolated from hearts of New Zealand white rabbits was measured in a glass chamber using Clark-type oxygen electrode. The cellular cyclic GMP levels, determined by radioimmunoassay, were increased by (1) adding 3-morpholinosydnonimine (SIN-1, 10(-8)-10(-5) M) and nitroprusside (10(-8)-10(-5) M), NO donors-soluble guanylate cyclase stimulators; (2) carbon monoxide (CO, 1.5 x 10(-8)-1.5 x 10(-6) M), soluble guanylate cyclase stimulator and (3) guanylin (10(-8)-10(-5) M), particulate guanylate cyclase stimulator. The baseline myocyte cyclic GMP level was 86 +/- 13 fmol/10(5) myocytes with a corresponding VO2 of 268 +/- 21 nl O2/min per 10(5) myocytes. An inverse relationship between cellular cyclic GMP levels and VO2 existed in these myocytes. The regression equations for the four treatments were: VO2 = -0.45 x [cyclic GMP] + 294.4, r = 0.94 for SIN-1; VO2 = -1.46 x [cyclic GMP] + 444.7, r = 0.96 for CO; VO2 = -1.25 x [cyclic GMP] + 389.1, r = 0.84 for guanylin and VO2 = -0.55 x [cyclic GMP] + 322.8. r = 0.79 for nitroprusside. The regression lines of the two NO donors were parallel. A similar result was also evident for the regressions of CO and guanylin. However, the slopes of both the SIN-1 and nitroprusside regression line were significantly less steep than that of either the CO or guanylin lines. Therefore, VO2 is reduced less for a similar increase in cyclic GMP with NO donors compared to direct stimulation with CO or guanylin. These results suggest that NO has metabolic effects on myocytes in addition to its stimulatory effects on cellular cyclic GMP.

Role of the NO-cGMP pathway in the muscarinic regulation of the L-type Ca2+ current in human atrial myocytes

1. The whole-cell patch-clamp technique was used to examine the participation of nitric oxide synthase (NOS) and soluble guanylyl cyclase in the muscarinic regulation of the L-type Ca2+ current (ICa) in freshly isolated human atrial myocytes. 2. Acetylcholine (ACh, 1 microM) decreased basal ICa by 39.1 +/- 5.5% (n = 8) under control conditions, and by 38.0 +/- 6.1% (n = 6) in the presence of 1H-[1,2,4]oxadiazolo[4,3-a]-quinoxaline-1-one (ODQ, 10 microM), a potent guanylyl cyclase inhibitor, and NG-monomethyl-L-arginine (L-NMMA, 1 mM), a competitive NOS inhibitor. L-NMMA alone had no effect on ICa, whilst ODQ increased ICa in 50% of the cells. 3. The accentuated antagonism of ACh on ICa, i.e. its ability to antagonize the stimulatory effect of beta-adrenergic agonists and, by extension, of other cAMP-elevating agents, was examined after the current was stimulated by either the beta-adrenergic agonist isoprenaline (Iso) or serotonin (5-HT). ACh (100 nM or 1 microM) completely blocked the stimulatory effects of 10 nM Iso or 10 nM 5-HT on ICa. 4. Extracellular application of Methylene Blue (MBlue, 10 microM), a guanylyl cyclase inhibitor, antagonized the inhibitory effect of 1 microM ACh on Iso- or 5-HT-stimulated ICa. However, this effect was overcome by a 100-fold higher ACh concentration and was not mimicked by an intracellular application of MBlue. 5. Inhibition of NOS and soluble guanylyl cyclase activities by addition of ODQ (10 microM) and L-NMMA (1 mM) to both extracellular and intracellular solutions, or by a 2 h pre-incubation of the cells with these inhibitors, modified neither the Iso (10 nM) response nor the inhibitory effect of ACh (100 nM or 1 microM) on Iso-stimulated ICa. 6. Extracellular application of the NO donor SNAP (S-nitroso-N-acetyl-D,L-penicillamine) at 100 nM produced a stimulatory effect on ICa in control conditions. This stimulatory effect was abolished by intracellular MBlue (20 microM) or by intracellular and extracellular application of ODQ (10 microM) in combination with L-NMMA (1 mM). 7. We conclude that the NO-cGMP pathway does not contribute significantly to the muscarinic regulation of ICa in human atrial myocytes.

Nitric oxide signaling mediates stimulation of L-type Ca2+ current elicited by withdrawal of acetylcholine in cat atrial myocytes

A perforated-patch whole-cell recording method was used to determine whether nitric oxide signaling participates in acetylcholine (ACh)-induced regulation of basal L-type Ca2+ current (ICa,L) in cat atrial myocytes. Exposure to 1 microM ACh for 2 min inhibited basal ICa,L (-21 +/- 3%), and withdrawal of ACh elicited rebound stimulation of ICa,L above control (80 +/- 13%) (n = 23). Stimulation of ICa,L elicited by withdrawal of ACh (but not ACh-induced inhibition of ICa,L) was blocked by either 50 microM hemoglobin; 30 microM ODQ or 10 microM methylene blue, inhibitors of soluble guanylate cyclase; 10 microM W-7, a calmodulin inhibitor; or 10 microM L-NIO, an inhibitor of constitutive NO synthase (NOS). In cells incubated in 5 mM L-arginine, ACh-induced rebound stimulation of ICa,L was enhanced compared with control responses. Histochemical assay (NADPH diaphorase) indicated that atrial myocytes express constitutive NOS. NO-donor, spermine/NO (SP/NO), >1 microM stimulated basal ICa,L. SP/NO-induced stimulation of ICa,L was inhibited by 50 microM hemoglobin, 30 microM ODQ, or 5 microM H-89, an inhibitor of PKA, and was unchanged by 50 microM MnTBAP, a peroxynitrite scavenger. When ICa,L was prestimulated by 10 microM milrinone, an inhibitor of cGMP-inhibited phosphodiesterase (type III) activity, SP/NO failed to further increase ICa,L. In cells incubated in pertussis toxin (3.4 microg/ml for 6 h; 36 degrees C), ACh failed to affect ICa,L, but 100 microM SP/NO or 10 microM milrinone still increased basal ICa,L. These results indicate that in cat atrial myocytes NO signaling mediates stimulation of ICa,L elicited by withdrawal of ACh but not ACh-induced inhibition of basal ICa,L. NO activates cGMP-induced inhibition of phosphodiesterase (type III) activity. Upon withdrawal of ACh, this mechanism allows cAMP to recover to levels above control, thereby stimulating ICa,L. Pertussis toxin-sensitive G-proteins couple M2 muscarinic receptors to NO signaling. NO-mediated stimulation of ICa, L elicited by withdrawal of ACh may be an important mechanism that rapidly restores cardiac pacemaker and contractile functions after cholinergic suppression of atrial activity.

Direct inhibition of expressed cardiac L-type Ca2+ channels by S-nitrosothiol nitric oxide donors

NO donors have complex effects on Ca2+ currents in native cardiac cells, with reports of direct stimulation and indirect cGMP-mediated inhibition or stimulation. To investigate the molecular basis of these effects, we tested the effects of one class of NO donors, S-nitrosothiols (RSNOs), on expressed cardiovascular L-type Ca2+ channels (alpha 1C +/- beta 1a +/- alpha 2 or alpha 1C +/- beta 2a +/- alpha 2) in human embryonic kidney (HEK293) cells. The RSNO compounds we used were S-nitroso-N-acetylpenicillamine (SNAP, 5 to 10 nmol/L or 100 to 800 mumol/L), S-nitrosocysteine (SNC, 100 mumol/L or 1 mmol/L), and S-nitrosoglutathione (GSNO, 1 mmol/L). Currents were measured using whole-cell patch recordings with 2 to 10 mmol/L Ba2+ as the charge carrier. SNAP reduced the amplitude of barium currents (IBa) through all the subunit combinations, with and EC50 of 360 mumol/L for alpha 1C + beta 1a channels. SNC or GSNO also inhibited IBa, albeit less potently. The inhibitory effect of SNAP was not affected by methylene blue (10 to 30 mumol/L) or 8-bromo-cGMP (200 to 400 mumol/L). The effects are relatively specific for Ca2+ channels, as expressed cardiac or skeletal muscle Na+ channels, which have a similar overall architecture, were barely affected by SNAP at concentrations as high as 1 mmol/L. We conclude that in the HEK293 expression system, the S-nitrosothiol NO donors inhibit L-type Ca2+ channels by a mechanism independent of cGMP.

Nitric oxide synthases and cardiac muscle. Autocrine and paracrine influences

The different cell types comprising cardiac muscle express one or more of the three isoforms (neuronal NOS, or nNOS; inducible NOS, or iNOS; and endothelial NOS, or eNOS) of nitric oxide synthase (NOS). nNOS is expressed in orthosympathetic nerve terminals and regulates the release of catecholamines in the heart. eNOS constitutively expressed in endothelial cells inhibits contractile tone and the proliferation of underlying vascular smooth muscle cells, inhibits platelet aggregation and monocyte adhesion, promotes diastolic relaxation, and decreases O2 consumption in cardiac muscle through paracrinally produced NO. eNOS is also constitutively expressed in cardiac myocytes from rodent and human species, where it autocrinally opposes the inotropic action of catecholamines after muscarinic cholinergic and beta-adrenergic receptor stimulation. iNOS gene transcription and protein expression are induced in all cell types after exposure to a variety of inflammatory cytokines. Aside from participating in the immune defense against intracellular microorganisms and viruses, the large amounts of NO produced autocrinally or paracrinally mediate the vasoplegia and myocardial depression characteristic of systemic immune stimulation and promote cell death through apoptosis. In cardiac myocytes, NO may regulate L-type calcium current and contraction through activation of cGMP-dependent protein kinase and cGMP-modulated phosphodiesterases. Other mechanisms independent of cGMP elevations may operate through interaction of NO with heme proteins, non-heme iron, or free thiol residues on target signaling proteins, enzymes, or ion channels. Given the multiplicity of NOS isoforms expressed in cardiac muscle and of the potential molecular targets for the NO produced, tight molecular regulation of NOS expression and activity at the transcriptional and posttranscriptional level appear to be needed to coordinate the many roles of NO in heart function in health and disease.

Cyclic GMP decreases cardiac myocyte oxygen consumption to a greater extent under conditions of increased metabolism

We tested the hypothesis that the negative effects of intracellular guanosine 3',5'-cyclic monophosphate (cyclic GMP) were more profound on cardiac myocyte oxygen consumption (VO2) during increased metabolism of the myocytes. The steady state VO2 of a suspension of single myocytes isolated from hearts of New Zealand White rabbits was measured in a glass chamber by using a Clark-type oxygen electrode, and cyclic GMP was determined by using a radioimmunoassay. The cellular cyclic GMP levels were increased either by adding 3-morpholino-sydnonimine (SIN-1), a guanylate cyclase stimulator, or zaprinast (ZAP), a cyclic GMP-phosphodiesterase inhibitor, at various doses. In 0.5 mM Ca2+ medium, average VO2 was 123 +/- 8 nl/min/100,000 cells, and cyclic GMP was 35.4 +/- 9.3 fmol/100,000 cells, and these increased significantly to 182 +/- 9 nl/min/100,000 cells and 78.2 +/- 7.3 fmol/100,000 cells in 2.0 mM Ca2+. There were dose-dependent responses of the VO2 and cellular cyclic GMP levels in responding to both SIN-1 and ZAP. An inverse relation between cellular cyclic GMP level and VO2 existed in the myocytes. The regression equations for the four treatments were log(VO2) = -0.002[cyclic GMP] + 2.19, r = 0.96 for SIN-1 in low (0.5 mM) Ca2+; log(VO2) = 0.005[cyclic GMP] + 1.80, r = 0.38 for ZAP in low Ca2+; log(VO2) = -0.001 [cyclic GMP] + 2.24, r = 0.82 for SIN-1 in high (2.0 mM) Ca2+; and log(VO2) = -0.004[cyclic GMP] + 2.56, r = 0.93 for ZAP in high Ca2+. The slope of ZAP regression line was significantly more negative than that of SIN-1 with high calcium. At any given level of cyclic GMP, ZAP decreased the VO2 to a greater extent than did SIN-1 although the latter caused the maximal increase in cyclic GMP level. The reduction in VO2 caused by a corresponding increase in cellular cyclic GMP was greater in myocytes incubated with high-Ca2+ medium.

cGMP-stimulated cyclic nucleotide phosphodiesterase regulates the basal calcium current in human atrial myocytes

EHNA (Erythro-9-[2-hydroxy-3-nonyl]adenine) is a wellknown inhibitor of adenosine deaminase. Recently, EHNA was shown to block the activity of purified soluble cGMPstimulated phosphodiesterase (PDE2) from frog, human, and porcine heart with an apparent Ki value of approximately 1 microM and with negligible effects on Ca2+/calmodulin PDE (PDE1), cGMP-inhibited PDE (PDE3), and low Km cAMP-specific PDE (PDE4) (Méry, P.F., C. Pavoine, F. Pecker, and R. Fischmeister. 1995. Mol. Pharmacol. 48:121-130; Podzuweit, T., P. Nennstiel, and A. Muller. 1995. Cell. Signalling. 7:733- 738). To investigate the role of PDE2 in the regulation of cardiac L-type Ca2+ current (ICa), we have examined the effect of EHNA on ICa in freshly isolated human atrial myocytes. Extracellular application of 0.1-10 microM EHNA induced an increase in the amplitude of basal ICa ( approximately 80% at 1 microM) without modification of the current-voltage or inactivation curves. The maximal stimulatory effect of EHNA on ICa was comparable in amplitude with the maximal effect of isoprenaline (1 microM), and the two effects were not additive. The effect of EHNA was not a result of adenosine deaminase inhibition, since 2'-deoxycoformycin (1-30 microM), another adenosine deaminase inhibitor with no effect on PDE2, or adenosine (1-10 microM) did not increase ICa. In the absence of intracellular GTP, the substrate of guanylyl cyclase, EHNA did not increase ICa. However, under similar conditions, intracellular perfusion with 0.5 microM cGMP produced an 80% increase in ICa. As opposed to human cardiomyocytes, EHNA (1-10 microM) did not modify ICa in isolated rat ventricular and atrial myocytes. We conclude that basal ICa is controlled by PDE2 activity in human atrial myocytes. Both PDE2 and PDE3 may contribute to keep the cyclic nucleotides concentrations at minimum in the absence of adenylyl and/or guanylyl cyclase stimulation.

Effects of the nitric oxide donor sodium nitroprusside on intracellular pH and contraction in hypertrophied myocytes

BACKGROUND

We compared the effects of the nitric oxide donor sodium nitroprusside (SNP) on intracellular pH (pHi), intracellular calcium concentration ([Ca2+]i) transients, and cell contraction in hypertrophied adult ventricular myocytes from aortic-banded rats and age-matched controls.

METHODS AND RESULTS

pHi was measured in individual myocytes with SNARF-1, and [Ca2+]i transients were measured with indo 1 simultaneously with cell motion. Experiments were performed at 37 degrees C in myocytes paced at 0.5 Hz in HEPES-buffered solution (extracellular pH = 7.40). At baseline, calibrated pHi, diastolic and systolic [Ca2+]i values, and the amplitude of cell contraction were similar in hypertrophied and control myocytes. Exposure of the control myocytes to 10(-6) mol/L SNP caused a decrease in the amplitude of cell contraction (72 +/- 7% of baseline, P < .05) that was associated with a decrease in pHi (-0.10 +/- 0.03 U, P < .05) with no change in peak systolic [Ca2+]i. In contrast, in the hypertrophied myocytes exposure to SNP did not decrease the amplitude of cell contraction or cause intracellular acidification (-0.01 +/- 0.01 U, NS). The cGMP analogue 8-bromo-cGMP depressed cell shortening and pHi in the control myocytes but failed to modify cell contraction or pHi in the hypertrophied cells. To examine the effects of SNP on Na(+)-H+ exchange during recovery from intracellular acidosis, cells were exposed to a pulse and washout of NH4Cl. SNP significantly depressed the rate of recovery from intracellular acidosis in the control cells compared with the rate in hypertrophied cells.

CONCLUSIONS

SNP and 8-bromo-cGMP cause a negative inotropic effect and depress the rate of recovery from intracellular acidification that is mediated by Na(+)-H+ exchange in normal adult rat myocytes. In contrast, SNP and 8-bromo-cGMP do not modify cell contraction or pHi in hypertrophied myocytes.

Blockade of nitric oxide synthesis reduces myocardial oxygen consumption in vivo

BACKGROUND

Although cardiac myocytes and coronary vascular endothelium are known to express a constitutive form of NO synthase, the in vivo effects of tonic endogenous production of NO on myocardial O2 consumption and contractile performance remain unclear.

METHODS AND RESULTS

The effects of blockade of NO synthase were determined in intact dogs. Myocardial O2 consumption decreased systematically over a wide range of hemodynamic demand after the systemic administration of N omega-nitro-L-arginine methyl ester (L-NAME) or N omega-nitro-L-arginine. Decreases after doses of 1 and 10 mg/kg L-NAME averaged 23 +/- 3.8% and 34 +/- 7.2% at a heart rate of 90 bpm in open-chest animals. Similar reductions occurred after the administration of L-NAME and N omega-nitro-L-arginine in chronically instrumented animals and were unaffected by beta-adrenergic blockade. Intracoronary infusion of L-NAME in chronically instrumented animals reduced both myocardial O2 consumption and regional segment shortening, even at a dose that did not increase systemic arterial pressure.

CONCLUSIONS

The blockade of NO synthesis reduces myocardial O2 consumption in vivo. The decrease in O2 consumption is accompanied by a decrease in segment shortening. It involves a direct myocardial action of NO, is unaffected by beta-blockade, and is consistent with in vitro studies indicating that low levels of NO augment contractile performance by inhibition of a cGMP-dependent phosphodiesterase.

Voltage-gated calcium channel currents in human coronary myocytes. Regulation by cyclic GMP and nitric oxide

Voltage-gated Ca2+ channels contribute to the maintenance of contractile tone in vascular myocytes and are potential targets for vasodilating agents. There is no information available about their nature and regulation in human coronary arteries. We used the whole-cell voltage-clamp technique to characterize Ca2+-channel currents immediately after enzymatic dissociation and after primary culture of coronary myocytes taken from heart transplant patients. We recorded a dihydropyridine-sensitive L-type current in both freshly isolated and primary cultured cells. A T-type current was recorded only in culture. The L- (but not the T-) type current was inhibited by permeable analogues of cGMP in a dose-dependent manner. This effect was mimicked by the nitric oxide-generating agents S-nitroso-N-acetylpenicillamine (SNAP) and 3-morpholinosydnonimine which increased intracellular cGMP. Methylene blue, known to inhibit guanylate cyclase, antagonized the effect of SNAP. Inhibitions by SNAP and cGMP were not additive and seemed to occur through a common pathway. We conclude that (a) L-type Ca2+ channels are the major pathway for voltage-gated Ca2+ entry in human coronary myocytes; (b) their inhibition by agents stimulating nitric oxide and/or intracellular cGMP production is expected to contribute to vasorelaxation and may be involved in the therapeutic effect of nitrovasodilators; and (c) the expression of T-type Ca2+ channels in culture may be triggered by cell proliferation.

Dobutamine enhances cardiodepressant effects of receptor-mediated coronary endothelial stimulation

BACKGROUND

In humans, intracoronary infusion of substance P reduces left ventricular end-systolic pressure and left ventricular peak systolic pressure because of earlier onset of left ventricular relaxation induced by paracrine myocardial action of mediators released from the coronary endothelium. The present study investigated in humans the effects of beta-adrenergic stimulation, which also induces earlier left ventricular relaxation, on the left ventricular myocardial contractile response to intracoronary infusion of substance P.

METHODS AND RESULTS

Data were obtained in 13 patients after cardiac transplantation and in 3 patients with dilated nonischemic cardiomyopathy. Microtip left ventricular pressure recordings were obtained during a 5-minute intracoronary infusion of substance P (20 pmol/min) under control conditions and then repeated during concurrent intravenous administration of dobutamine. In the presence of dobutamine, intracoronary substance P caused a greater fall in left ventricular end-systolic pressure (transplantation control, -9 +/- 11 versus transplantation dobutamine, -20 +/- 18 mm Hg [P < .05]; cardiomyopathy control, -4 +/- 1 versus cardiomyopathy dobutamine, -10 +/- 3 mm Hg [P < .05]) and in left ventricular peak systolic pressure (transplantation control, -14 +/- 10 versus transplantation dobutamine, -30 +/- 22 mm Hg [P < .01]; cardiomyopathy control, -9 +/- 7 versus cardiomyopathy dobutamine, -15 +/- 6 mm Hg [P = .1]).

CONCLUSIONS

Dobutamine enhances the cardiodepressant effect on myocardial contractile performance of receptor-mediated coronary endothelial stimulation in transplant recipients and in patients with dilated nonischemic cardiomyopathy. This enhancement could result from a potentiating interaction of the relaxation-hastening effect exerted by beta-adrenergic stimulation and by mediators released from the coronary endothelium, such as nitric oxide.

Muscarinic regulation of the L-type calcium current in isolated cardiac myocytes

Muscarinic agonists regulate the L-type calcium current in isolated cardiac myocytes. The second messengers pathways involved in this regulation are discussed briefly, with particular emphasis on the involvement of cAMP and cGMP pathways.

Paracrine coronary endothelial modulation of diastolic left ventricular function in man: implications for diastolic heart failure

Coordinated release of relaxing and contracting factors from the endothelium modulates arterial distensibility. Recently, a similar release of the same and other factors from the coronary endothelium was shown to modulate myocardial performance in humans. This paracrine modulation of left ventricular (LV) performance by substances released from the coronary endothelium mainly affects diastolic LV function. This was evident from the reduction in end-systolic LV pressure, the earlier onset of LV relaxation and the increased LV diastolic distensibility observed in normal subjects during bi-coronary infusion of substance P. In experimental preparations, substance P elicited similar effects on diastolic LV function, which were attributed to a paracrine myocardial action of nitric oxide (NO) because they were absent after addition of hemoglobin. In normal subjects, the myocardial effects of NO were investigated during bi-coronary infusion of the NO-donor sodium nitroprusside and resembled the effects observed during bi-coronary infusion of substance P. This paracrine control of diastolic LV function by the coronary endothelium is influenced by substrate availability and by many neurohumoral substances, whose plasma levels are raised in heart failure. In transplant recipients, bi-coronary co-infusion of substance P and of L-arginine, the substrate for NO production, potentiated the fall in LV filling pressures. Pretreatment with intravenous dobutamine augmented the drop in LV end-systolic pressures observed during bi-coronary infusion of substance P. In isolated papillary muscles, a higher baseline myocardial c-GMP level, as induced by atrial natriuretic peptide, potentiates the negative inotropic and relaxation hastening effects of NO. In isolated ejecting guinea-pig hearts, an endothelin receptor antagonist improved diastolic LV function and this improvement implies paracrine myocardial action on diastolic LV function not only of NO but also of endothelin. Coronary endothelial control of myocardial function affects LV performance both acutely and chronically. An acute increase in heart rate augments release of NO because of coronary reactive hyperemia, lowers LV filling pressures thereby promoting subendocardial perfusion, and hastens LV relaxation thereby prolonging the diastolic time interval for coronary perfusion. Chronic changes in coronary endothelial function could also influence diastolic LV performance. Enhanced coronary endothelial NO release, as occurs during chronic exercise or pacing, could explain increased LV diastolic distensibility observed in athlete's heart and in tachycardia cardiomyopathy. Reduced endothelial NO release, as occurs with aging or after transplantation, could contribute to reduced LV diastolic distensibility in the elderly or in allograft recipients.

Elucidating molecular mechanisms of septic cardiomyopathy--the cardiomyocyte model

In the multiple organ dysfunction syndrome of sepsis and septic shock the heart is one of the organs subject to failure. Many new insights into the mechanisms underlying septic cardiomyopathy were gained in the last years. Experimental work with neonatal and adult cardiomyocytes considerably contributed to this progress, facilitating the documentation of direct attenuation of the contractions of the heart muscle cell by toxins and mediators, as well as investigating the underlying cellular mechanisms. With this respect, contractile-depressant effects have been found in cardiomyocytes for many toxins and sepsis mediators, with endotoxin, Pseudomonas exotoxin A, tumor necrosis factor alpha, interleukin-1 and nitric oxide being the most relevant ones identified. These substances interfere at clinically relevant concentrations with several main inotropic axes, not only with the beta-adrenoceptor/adenylyl cyclase and with the NO-cGMP-system-on which most of the interest is focused at present-but also with the alpha 1-adrenoceptor/phosphoinositide pathway and the Ca2+ homeostasis of the cardiomyocyte, the latter representing the common final inotropic pathway. Not a single cardiodepressant factor, but more likely a total bunch of toxins and mediators with different attack mechanisms seem to contribute to the picture of septic cardiomyopathy.

Alterations in inotropy, nitric oxide and cyclic GMP synthesis, protein phosphorylation and ADP-ribosylation in the endotoxin-treated rat myocardium and cardiomyocytes

To evaluate the effects of the in vivo endotoxin treatment of the rat on (1) the contractile responses in the subsequently isolated papillary muscle to adrenergic and cholinergic agonists and (2) the biochemical parameters (cyclic GMP, nitric oxide synthesis, protein phosphorylation and ADP-ribosyslation) in the subsequently isolated cardiomyocytes. Following the in vivo endotoxin treatment (4 mg/kg i.p., 18 h), contractile responses to increasing amounts of isoprenaline or to increasing amounts of oxotremorine in the presence of a fixed amount of isoprenaline were determined in isolated papillary strips. Activities of nitric oxide synthase, guanylyl cyclase, as well as phosphorylation of phospholamban and troponin-inhibitory subunit, and pertussis toxin-catalyzed and endogenous ADP-ribosylations were determined in the intact cardiomyocytes and subcellular fractions. The increase in the force of contraction by isoprenaline was reduced, while its inhibition by oxotremorine was greater in the endotoxin-treated papillary strips. The activities of both nitric oxide synthase, primarily of the inducible form of the enzyme, and cytosolic guanylyl cyclase were higher while the phosphorylations of both phospholamban and troponin-inhibitory subunit were of lesser magnitude in the cardiomyocytes following the in vivo endotoxin treatment. Pertussis toxin-catalyzed ADP-ribosylation of the 41 kDa polypeptide, which is the alpha subunit of Gi, was also decreased. The results of the present study support the postulate that alterations in both the cyclic AMP and cyclic GMP signalling cascade contribute to the myocardial dysfunction caused by endotoxin and cytokines.

Negative inotropic actions of nitric oxide require high doses in rat cardiac muscle

Initial experiments were designed to determine if vasoactive concentrations of nitric oxide (NO) alter contractility in rat heart. Contractile function was monitored in left atrial and papillary muscles (30 degrees C; paced at 0.5 Hz) during cumulative addition of 3-morpholino-sydnonimine-HCl(SIN-1), an agent that releases NO. At concentrations between 10(-7) and 10(-4) M (NO concentrations of approximately 10(-8)- 3 x 10(-7) M), SIN-1 did not affect contractility in either tissue. Similarly, 10(-4) M SIN-1 did not alter the positive inotropic responses to isoproterenol or increasing extracellular [Ca+2] ([Ca+2]o). To obtain higher concentrations of NO, additional studies were conducted using authentic NO. NO-saturated stock solutions and a corresponding control solvent were adjusted to pH 1.6 with HCl. Dose-dependent effects of NO were examined by adding aliquots of the stock solutions (or control solvent) to the bathing solution. At final concentrations of 1 x 10(-5)- 5 x 10(-4) M, NO produced transient, concentration-dependent decreases in contractility that were paralleled by reductions in buffer pH. Control solvent elicited similar reductions in pHo and transient decreases in contractility; however, the negative inotropic action elicited by the NO-containing solution was approximately 20% greater than that observed in control conditions. These data demonstrate that only high concentrations of NO depress contractility in isolated rat cardiac muscle, and suggest that this effect is mediated by both acidosis and a pHo-independent mechanism.

Chronic beta 1-adrenoceptor blockade sensitises the H1 and H2 receptor systems in human atrium: rôle of cyclic nucleotides

We have reported that chronic treatment of patients with beta 1-adrenoceptor blockers sensitises isolated atrial preparations to adrenaline, noradrenaline and 5-Ht. We have now examined the effect of chronic treatment with beta-adrenoceptor blockers on responses to histamine of human right atrial appendages. We compared the effects of histamine on contractile force, cyclic AMP and cyclic GMP levels as well as cyclic AMP-dependent protein kinase (PKA) activity and explored the arrhythmogenic effects of histamine in preparations obtained from patients chronically treated or not treated with beta-adrenoceptor blockers. Histamine increased contractile force in paced preparations; the effects were blocked by the H2 receptor antagonist famotidine (0.1-30 mumol/l). The maximum inotropic response to histamine was doubled and the inotropic potency of histamine 0.4 log units greater in atria from beta-adrenoceptor blocker-treated compared to non beta-adrenoceptor blocker-treated patients. Histamine elicited frequency-dependent arrhythmias that were blocked by famotidine (30 mumol/l) but not by mepyramine (1 mumol/l). The incidence of arrhythmias was higher in atria from beta-adrenoceptor blocker-treated compared to untreated patients. Histamine increased both cyclic AMP and cyclic GMP levels, as well as PKA activity, significantly more in atria from beta-adrenoceptor blocker-treated compared to those from untreated patients. Mepyramine 1 mumol/l prevented the histamine-evoked increase in cyclic GMP levels, reduced the inotropic hyperresponsiveness and abolished the hyperresponsiveness in cyclic AMP levels and PKA activity observed in patients chronically treated with beta blockers. Sodium nitroprusside 10 mumol/l caused smaller increase of cyclic GMP levels than histamine and restored the contracile force depressed by mepyramine to its original level in atria from beta-adrenoceptor blocker-treated patients. The evidence is consistent with sensitisation of both the histamine H1 and histamine H2 receptor systems by chronic beta 1-adrenoceptor blockade. H1 receptor-mediated increases in cyclic GMP, enhanced through an as yet unknown mechanism by chronic beta 1-adrenoceptor blockade, may inhibit phosphodiesterase 3 activity, thereby causing enhanced histamine-evoked increases in cyclic AMP levels and PKA activity, and accounting partially for the increased inotropic responses to histamine through H2 receptors.

Regulation of myocardial calcium channels by cyclic AMP metabolism

Hormonal regulation of cardiac inotropism is often correlated with modification of the L-type Ca-channel current. Among several regulatory pathways that control Ca-channel activity, the best described one is the cAMP cascade. Cyclic AMP-dependent phosphorylation of the Ca-channel results in an increase of the mean open probability of the individual Ca-channels and, thus, of the macroscopic Ca current. Modulation of cAMP concentration can take place at the level of adenylyl cyclases or cAMP phosphodiesterases. Of major interest is the fact that the activity of two different forms of phosphodiesterases is controlled by the level of intracellular cGMP. Thus, cAMP metabolism is intimately associated with cGMP metabolism, and both determine the degree of cAMP-dependent phosphorylation of cardiac Ca-channels. This brief discussion will focus on these two levels of control and their relative importance in the cAMP-dependent regulation of myocardial Ca-channels.