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Carbachol augments Na/Ca exchange current via M_(2) muscarinic receptors in guinea pig ventricular myocytes

Cui Xiangli, Chen Huanzhen, Wu Dongmei, Wu Bowei

Departmentn of Physiology,Shanxi Medical University.Taiyuan 030001,Shanxi

Abstract

Stimulation of cardiac mAChRs by carbachol (CCh) produces a biphasic inotropic response. The mechanisms of the positive inotropic response by higher concentration of CCh appear to be paradoxical. This article was aimed to study the mechanism of the positive inotropic effect of CCh in guinea pig ventricular myocytes. The effects of CCh on L-type calcium current (I_(Ca)) and Na/Ca exchange current (I_(Na/Ca)) were observed in voltage-clamped guinea pig ventricular myocytes by using Axon 200A amplifier. The results showed that CCh (100 #mu#mol/L) increased both forword mode and reverse mode INa/Ca from (1.2±0.1) pA/pF to (2.0±0.3) pA/pF for forward mode (P＜0.01) and from (1.3+0.5) pA/pF to (2.1+0.8) pA/pF for reverse mode (P＜0.01), respectively. CCh had no effect on I_(Ca). The stimulating effect of CCh on I_(Na/Ca) could be blocked by application of atropine, a nonselective blocker of muscarinic receptors,which means that the stimulating effect of CCh is through the activation of muscarinic receptors. We made a further study by using methoctramine, a selective antagonist of M_(2) muscarinic receptors. It completely abolished I_(Na/Ca) induced by 100 #mu#mol/L CCh, indicating that the effect of CCh on I_(Na/Ca) was mediated by M_(2) muscarinic receptors. It is generally accepted that contraction in cardiac myocytes results from elevation of intracellular Ca~(2+) concentration. Ca~(2+) enters the cells through two pathways: L-type Ca~(2+) channels and, less importantly, reverse mode Na/Ca exchange. The calcium influx via both pathways promotes the contraction of cardiac myocytes.Because CCh had no effect on L-type Ca~(2+) current, the increase in Na/Ca exchange current might be the main factor in the positive inotropism of CCh. These results suggest that the positive inotropic effect of CCh in guinea pig heart is through stimulation of Na/Ca exchange and is mediated by M_(2) muscarinic receptors.

Key words: carbachol;muscarinic receptor;Na-Ca exchanger;methoctramine;Guinea pig;cardiomegaly

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Citing This Article：

Cui Xiangli, Chen Huanzhen, Wu Dongmei, Wu Bowei. Carbachol augments Na/Ca exchange current via M_(2) muscarinic receptors in guinea pig ventricular myocytes. Acta Physiol Sin 2004; 56 (6): (in Chinese with English abstract).