Effect Of C-Type Natriuretic Peptide On Delayed Rectifier Potassium Currents In Gastric Antral Myocytes Of Guinea Pigs

Since C-type natriuretic peptide (CNP) was found by Soudh et al in 1990, the physiological effect of CNP has been known by more and more scholars. CNP has been found to have a regulatory effect on the central nervous system, the cardiovascular system and the gastrointestinal tract. CNP could inhibit the basal secretion of arginine-vasopressin (AVP) from rat supraoptic nucleus (SON) neurons through natriuretic peptide receptor NPR-B and could have a role in the body water and electrolyte homeostasis in the central nervous system. CNP could cause local vasodilation, inhibit the spontaneous motility of gastric antral circular smooth muscles in humans, rats and guinea-pigs, and prevent the proliferation of smooth muscle cells and the chondrocytes. The effect of CNP on the relaxation of the vascular smooth muscle was mediated by activating the large-conductance calcium-activated potassium channels through cyclic GMP signaling pathway. However, the ionic mechanism of the CNP-induced inhibition of the motility of the gastric antral smooth muscles is still unclear, and the clarification of this problem is of theoretical significance in the study of gastrointestinal motility.As one of the important signal transduction systems in the cell membrane, the potassium channels played a major role in the regulation of cellular excitability and contractility. In our previous studies, we have demonstrated that two types of potassium currents were in the guinea pig gastric myocytes, i.e. the delayed rectifier potassium currents [I_K(v)] and the Ca~2+-activated potassium currents [I_K(Ca)]. CNP increased I_K(ca) in the guinea pig gastric antral circular myocytes and in other smooth muscle cells. However, the effect of CNP on I_K(v) and its mechanism was still unclear in guinea-pig gastric antral myocytes. Therefore, the aim of this study was to observe the role of CNP on I_K(v) and to clarify its mechanism by employing the conventionalwhole-cell patch clamp technique in the single gastric antral myocytes isolated by collagenase in the guinea-pig. The results were as follows:1. In the whole-cell configuration, membrane potential was clamped at -60 mV. When a pipette solution containing egtazic acid 10 mM, 7k(V) was elicited by a step voltage command pulse from -40 mV to +80mV for 440ms with a 20mV increment at 10-second intervals in the single gastric antral myocytes. The mean amplitude of 7k( was921.84±33.49pAat+60mV (n = 60) .2. CNP markedly inhibited /mvjin a dose-dependant manner. CNP inhibited from 100% of control to 86.11±1.63% (n=7), 78.37±2.62% (n=10) and 67.71±2.34% (n=14) at 0.01|iM, 0.1 [iM and IujviCNP at +60mV respectively. There was a significant difference between the control group and each CNP-treated group respectively, as well as a significant difference between each of the CNP-treated groups.3. To explore the mechanism of the CNP-induced inhibition of /|?v), further experiments have also been performed with the following results:1) LY83583-pretreated: Using O.OluM LY83583, an inhibitor of guanylate cyclase to decrease the generation of cGMP, then the effect of CNP on /k(v> was observed again. It was noticed that LY83583 itself had no effect on 7j<(V)(7:>>0.05, n=7), but the CNP-induced inhibition of /]<(V)Was significantly impaired by LY83583. The inhibition percentage of CNP on /K(V) was 78.37±2.62% and 89.51±4.52% at +60mV before and after pretreatment with LY83583 respectively ( 7*0.05, n=7).2) Zaprinast-pretreated: Using 0.1 (.im Zaprinast, a cGMP-sensitive phosphoesterase (PDE) inhibitor to increase the generation of cGMP , then the effect of CNP on 7k(v) was observed again. It was noticed that Zaprinast itself had no effect on /k(V) (T^O.05, n=7), but significantly potentiated the CNP-induced inhibition of 7k(v). The inhibition percentage of CNP on /KlV) was 78.37±2.62% and 70.30±2.23% at +60 mV before and after pretreatment with Zaprinast respectively (P <0.05, n=8).3) KT5823-pretreated: Using Ium KT5823, an inhibitor of cGMP- dependent protein kinase (PKG) to block the cGMP- PKG pathway , then the effect of CNP on/k(V) was observed again. It was noticed that KT5823 itself had no effect on Ik(V){P >0.05, n=9), but completely blocked the CNP-induced inhibition of/K(v>(n=9).The results suggested that 1) CNP inhibited the delayed rectifier potassium currents in a dose-dependent manner and that 2) the CNP-induced inhibition of the delayed rectifier potassium currents was mediated by cGMP-PKG pathway in the gastric antral myocytes of the guinea pig.