Effects of calcitonin gene-related peptide on membrane currents in mammalian cardiac myocytes

The Role of Calcitonin Gene Related Peptide (CGRP) in Neurogenic Vasodilation and Its Cardioprotective Effects

Calcitonin gene-related peptide (CGRP) is a highly potent vasoactive peptide released from sensory nerves, which is now proposed to have protective effects in several cardiovascular diseases. The major α-form is produced from alternate splicing and processing of the calcitonin gene. The CGRP receptor is a complex composed of calcitonin like receptor (CLR) and a single transmembrane protein, RAMP1. CGRP is a potent vasodilator and proposed to have protective effects in several cardiovascular diseases. CGRP has a proven role in migraine and selective antagonists and antibodies are now reaching the clinic for treatment of migraine. These clinical trials with antagonists and antibodies indicate that CGRP does not play an obvious role in the physiological control of human blood pressure. This review discusses the vasodilator and hypotensive effects of CGRP and the role of CGRP in mediating cardioprotective effects in various cardiovascular models and disorders. In models of hypertension, CGRP protects against the onset and progression of hypertensive states by potentially counteracting against the pro-hypertensive systems such as the renin-angiotensin-aldosterone system (RAAS) and the sympathetic system. With regards to its cardioprotective effects in conditions such as heart failure and ischaemia, CGRP-containing nerves innervate throughout cardiac tissue and the vasculature, where evidence shows this peptide alleviates various aspects of their pathophysiology, including cardiac hypertrophy, reperfusion injury, cardiac inflammation, and apoptosis. Hence, CGRP has been suggested as a cardioprotective, endogenous mediator released under stress to help preserve cardiovascular function. With the recent developments of various CGRP-targeted pharmacotherapies, in the form of CGRP antibodies/antagonists as well as a CGRP analog, this review provides a summary and a discussion of the most recent basic science and clinical findings, initiating a discussion on the future of CGRP as a novel target in various cardiovascular diseases.

Localization of calcitonin gene-related peptide in cardiomyocytes: comparison of neonatal and dedifferentiating cells to adult myocytes

The purpose of this study was to localize sites of calcitonin gene-related peptide binding in neonatal, freshly isolated and dedifferentiated adult cardiac myocytes in order to help us elucidate the mechanisms of action of this neuropeptides. Previous work has shown that treatment with calcitonin gene-related peptide results in dramatic changes in calcium transients, so we carried out multi-channel acquisitions of fluorescently labeled images to reveal where calcitonin gene-related protein and the L-type calcium channel were localized. Calcitonin gene-related protein was sparse and randomly distributed in rod-like adult cardiomyocytes, found in abundance in areas of the cell where striations were apparent and not where adhesion proteins predominated in dedifferentiating adult myocytes, and in a large perinuclear concentration, with some spreading into the cytoplasm in neonatal cells. Subsequent modeling demonstrated that calcitonin gene-related peptide and the L-type calcium channel protein were closely associated in each of the three myocyte types, suggesting that while the peptide has dramatic and different effects on intracellular calcium levels of the various cardiomyocytes, the action is probably via diverse mechanisms as a result of effects on different channels or pump proteins due to alterations in intracellular calcium concentrations.

Calcitonin gene-related peptide in vivo positive inotropy is attributable to regional sympatho-stimulation and is blunted in congestive heart failure

Calcitonin gene-related peptide (CGRP) is a nonadrenergic/noncholinergic (NANC) peptide with vasodilatative/inotropic action that may benefit the failing heart. However, precise mechanisms for its in vivo inotropic action remain unclear. To assess this, dogs with normal or failing (sustained tachypacing) hearts were instrumented for pressure-dimension analysis. In control hearts, CGRP (20 pmol/kg per minute) enhanced cardiac contractility (eg, +33+/-4.2% in end-systolic elastance) and lowered afterload (-14.2+/-2% in systemic resistance, both P<0.001). The inotropic response was markedly blunted by heart failure (+6.5+/-2%; P<0.001 versus control), whereas arterial dilation remained unaltered (-19.3+/-5%). CGRP-positive inotropy was not attributable to reflex activation because similar changes were observed in the presence of a ganglionic blocker. However, it was fully prevented by the beta-receptor antagonist (timolol), identifying a dominant role of sympatho-stimulatory signaling. In control hearts, myocardial interstitial norepinephrine assessed by microdialysis almost doubled in response to CGRP infusion, whereas systemic plasma levels were unchanged. In addition, CGRP receptors were not observed in ventricular myocardium but were prominent in coronary arteries and the stellate ganglia. Ventricular myocytes isolated from normal and failing hearts displayed no inotropic response to CGRP, further supporting indirect sympatho-stimulation as the primary in vivo mechanism. In contrast, the peripheral vasodilatative capacity of CGRP was similar in femoral vascular rings from normal and failing hearts in dogs. Thus, CGRP-mediated positive inotropy is load-independent but indirect and attributable to myocardial sympathetic activation rather than receptor-coupled stimulation in canine hearts. This mechanism is suppressed in heart failure, so that afterload reduction accounts for CGRP-enhanced function in this setting.

Troglitazone inhibits voltage-dependent calcium currents in guinea pig cardiac myocytes

BACKGROUND

It has been suggested that intracellular Ca2+ overload in cardiac myocytes leads to the development of diabetic cardiomyopathy. Troglitazone, an insulin-sensitizing agent, is a promising therapeutic agent for diabetes and has been shown to prevent diabetes-induced myocardial changes. To elucidate the underlying mechanism of troglitazone action on cardiac myocytes, the effects of troglitazone on voltage-dependent Ca2+ currents were examined and compared with classic Ca2+ antagonists (verapamil and nifedipine).

METHODS AND RESULTS

Whole-cell voltage-clamp techniques were applied in single guinea pig atrial myocytes. Under control conditions with CsCl internal solution, the voltage-dependent Ca2+ currents consisted of both T-type (ICa,T) and L-type (ICa,L) Ca2+ currents. Troglitazone effectively reduced the amplitude of ICa,L in a concentration-dependent manner. Troglitazone also suppressed ICa,T, but the effect of troglitazone on ICa,T was less potent than that on ICa,L. The current-voltage relationships for ICa,L and the reversal potential for ICa,L were not altered by troglitazone. The half-maximal inhibitory concentration of troglitazone on ICa,L measured at a holding potential of -40 mV was 6.3 micromol/L, and 30 micromol/L troglitazone almost completely inhibited ICa,L. Troglitazone 10 micromol/L did not affect the time courses for inactivation of ICa,L and inhibited ICa,L mainly in a use-independent fashion, without shifting the voltage-dependency of inactivation. This effect was different from those of verapamil and nifedipine. Troglitazone also reduced isoproterenol- or cAMP-enhanced ICa,L.

CONCLUSIONS

These results demonstrate that troglitazone inhibits voltage-dependent Ca2+ currents (T-type and L-type) and then antagonizes the effects of isoproterenol in cardiac myocytes, thus possibly playing a role in preventing diabetes-induced intracellular Ca2+ overload and subsequent myocardial changes.

Antiarrhythmic effect and its underlying ionic mechanism of 17beta-estradiol in cardiac myocytes

1. The effects of oestrogens on action potential and membrane currents were examined in single guinea-pig atrial myocytes. 2. 17Beta-estradiol (3-10 microM) shortened the action potential duration without significant changes in the resting membrane potential. E-4031 (1 microM) markedly prolonged the action potential duration and induced early afterdepolarization, and 17beta-estradiol (10 microM) abolished it. 3. When cells were perfused in isoproterenol-containing solution, action potentials due to abnormal automaticity caused by membrane depolarization developed, and were also inhibited by 17beta-estradiol. 4. Under voltage clamp conditions, the voltage-dependent Ca2+ currents consisted of both T-(I(Ca,T)) and L-type (I(Ca,L)). 17Beta-estradiol reduced I(Ca,L) concentration-dependently, while it (10 microM) suppressed I(Ca,T) only by approximately 10%. 17Beta-estradiol did not affect time courses of I(Ca,L) inactivation, but it shifted the steady-state inactivation curve to more negative potentials. 5. 17Beta-estradiol (10 microM) did not affect the time-dependent K+ current (I(K)), referred to as I(Kr) and I(Ks) and inwardly rectifying K+ current. However, 17beta-estradiol (30 microM) or diethylstilbestrol (10 microM) inhibited K+ currents. 6. DES and ethinylestradiol (EES) also suppressed I(Ca,L), but testosterone and progesterone failed to inhibit I(Ca,L) The potency of the inhibitory effect on I(Ca,L) was DES> EES> 17beta-estradiol. 7. 17Beta-estradiol and DES also inhibited the cyclic AMP-enhanced I(Ca,L), but cyclic GMP in the pipette or pretreatment of L-NAME could not block the effects of oestrogen on I(Ca,L). 8 These results suggest that oestrogen specifically has antiarrhythmic effects, possibly by acting the L-type Ca2+ channels. The antiarrhythmic effects of oestrogens may contribute to the cardioprotective actions of oestrogens.

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.

The characteristics in the inhibitory effects of capsaicin on voltage-dependent K(+) currents in rat atrial myocytes

The electrophysiological effects of capsaicin in rat atrial myocytes were examined. Measurement of contractile force was done in rat left atria. Whole-cell patch-clamp technique was primarily used to study the change in membrane potential and ionic currents. Capsaicin produced an initial rise and a sustained increase in contractile force in rat left atria. Capsaicin (10 μM) caused a significant prolongation of atrial action potential. In voltage-clamp experiments, capsaicin (1-100 μM) caused the reversible reduction in the amplitude of transient outward (I(TO)) and late outward (I(L)) K(+) currents in concentration- and voltage-dependent manners. The time course for inactivation of I(TO) was changed to the biexponential process after the application of capsaicin. Capsaicin failed to cause any significant shift in quasi-steady-state inactivation curve of I(TO). The EC(50) values for the inhibitory effects of capsaicin on I(TO) and I(L) were 5 and 20 μM, respectively. Capsaicin also suppressed the amplitude of acetylcholine- or adenosine-induced K(+) current, i.e., I(K(ACh,Ado)). The EC(50) value for capsaicin-mediated inhibition of I(K(ACh,Ado)) is 50 μM. The present findings suggest that in isolated rat atria, during capsaicin exposure, the capsaicin-mediated inhibition of these K(+) channels is one of the ionic mechanisms underlying the positive inotropic and chronotropic actions.

Effect of capsaicin on membrane currents in cultured vascular smooth muscle cells of rat aorta

The application of capsaicin (1 microM) produced a minor relaxant effect in endothelium-denuded rat aortae. However, capsaicin caused a greater relaxation of blood vessels precontracted with high K+ or phenylephrine. The effects of capsaicin on the ionic currents were also examined in A7r5 vascular smooth muscle cells. The tight-seal whole-cell voltage clamp technique was used. Capsaicin inhibited the Ba2+ inward current (IBa) through the voltage-dependent L-type Ca2+ channel in a concentration-dependent fashion, whereas calcitonin gene-related peptide and phenylephrine produced a minor increase in IBa. Capsaicin did not alter the overall shape of current-voltage relationship of IBa. However, capsaicin (3 microM) shifted the quasi-steady-state inactivation curve of IBa to more negative membrane potential by about 5 mV. These effects of capsaicin on IBa were reversible. In addition, capsaicin had inhibitory effects on voltage dependent K+ currents. These results suggest that inhibition of the voltage-dependent L-type Ca2+ channel is involved in the capsaicin-induced relaxation of the vascular smooth muscle, whereas capsaicin-induced inhibition of voltage-dependent K+ channels might produce an increase in cell excitability.

The effect of calcitonin gene-related peptide on ischemic reperfusion-induced arrhythmias in rats

Thirty-six wistar rats (150-250 g body weight) were randomly divided into two groups. There were equal numbers of male and female rats in each group. Each group of rats was given either 0.5 ml normal saline or calcitonin gene-related peptide (CGRP, 20 micrograms/kg dissolved in 0.5 ml normal saline) intravenously within 5 min. Coronary arteries of these rats were occluded 5 min after administration. The coronary artery was then released from ligation 5 min later. The heart beat of all rats was monitored for 30 min. An electrocardiogram was recorded using limb leads and below-xiphoid leads. There was no ischemic reperfusion-induced arrhythmia in the first minute of reperfusion in CGRP group, while 13 rats occurred ischemic reperfusion-induced arrhythmia (ventricular tachycardia and fibrillation) in the normal saline group (P < 0.05). Average onset time of ischemic reperfusion-induced arrhythmia was 377.5 +/- 141.0 s/sim. In the CGRP group, compared with 42.7 +/- 55.4 s/sim. for the normal saline group (P < 0.01). The duration of ischemic reperfusion-induced arrhythmia was 15.0 +/- 30.6 s/sim. In the CGRP group compared with 104.4 +/- 143.8 s/sim. in the normal group (P < 0.05). There was no death in the CGRP group, but six rats in the normal saline group died (P < 0.05). The results showed that CGRP could reduce and delay the occurrence of ischemic reperfusion-induced arrhythmia such as ventricular tachycardia and fibrillation, especially in the early period, and decrease the mortality. The mechanism is not clear and needs to be studied further.

Relaxin increases heart rate by modulating calcium current in cardiac pacemaker cells

Relaxin (RLX), a reproductive hormone of the insulin family, increases heart rate in experimental animals. The cellular and ionic mechanisms responsible for this positive chronotropic effect remain unknown. We have investigated the actions of RLX on the action potential and underlying transmembrane ionic currents in single sinoatrial node cells of the rabbit heart under whole-cell voltage-clamp conditions, using both nystatin-perforated-patch and membrane-ruptured techniques. In this preparation RLX (0.8 to 80 nmol/L) caused reversible increases in the rate of spontaneous action potentials and a dose-dependent increase in the L-type calcium current, ICa(L). The best-fit Langmuir relation for the augmentation of ICa(L) yielded a threshold concentration of 1 nmol/L and a KD of 14 nmol/L. These effects of RLX appear to be mediated by increases in intracellular cyclic AMP (cAMP), since RLX was without effect after application of (1) the beta-adrenergic agonist isoprenaline (1 mumol/L) or (2) superfusion of the intracellular second messenger cAMP (100 mumol/L) or 8-Br-cAMP (100 to 200 mumol/L). Internal dialysis with an inhibitor of cAMP-dependent protein kinase (PKI, 7 mumol/L) abolished the effects of RLX. These results provide the first electrophysiological evidence that RLX modulates heart rate and contractility by increasing ICa(L) and suggest that the biochemical mechanism involves the formation of cAMP and activation of cAMP-dependent protein kinase.

Reversal of the inhibitory effects of calcitonin gene-related peptide (CGRP) and amylin on insulin secretion by the 8-37 fragment of human CGRP

The 8-37 fragment of human calcitonin gene-related peptide [(8-37)hCGRP] antagonizes the effects of calcitonin gene-related peptide (CGRP) and amylin in a number of tissues. We have studied the influence of (8-37)hCGRP on the effects of both CGRP and amylin on insulin secretion. In the perfused rat pancreas, homologous CGRP and amylin, at 75 pM, exerted comparable inhibitory effects on the insulin response to 9 mM glucose (ca. 70%; P < 0.025). These effects were antagonized by (8-37)hCGRP (1 microM). Our results suggest that CGRP and amylin act on the B-cell, at least in part, through a common receptor.

Calcitonin gene-related peptide: multiple actions, multiple receptors

Calcitonin gene-related peptide (CGRP) shows diversity both in its effects and its receptors. It is likely to have roles as a neurotransmitter, neuromodulator, local hormone and trophic factor. Its effects include rapid changes in neuronal activity, relaxation of many types of smooth muscle, actions on metabolism and changes in gene expression. Receptor heterogeneity has been revealed from experiments comparing agonist potency ratios and antagonist affinities. The evidence from these approaches is reviewed in this article and a speculative receptor classification scheme is proposed. Some of the likely future directions for CGRP research are discussed.