| The natriuretic peptides family has been known to be a kind of polypeptide with many bioactivities. ANP, the first member in the natriuretic peptides family, was first found by de Bold AJ, et al, in 1981. After further reseach on the natriuretic peptides family, up to now, the members in the natriuretic peptides include atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP), C-type natriuretic peptide (CNP), dendroaspis natriuretic peptide (DNP), micrurus natriuretic peptide (MNP) and ventricular natriuretic peptide (VNP). There have been three natriuretic peptides receptors (NPR), NPR-A, NPR-B and NPR-C, which have bonded with natriuretic peptides to exert their bioactivities. CNP was first found in porcine brain by Sudoh, et al, in 1990, and it was a peptide with 22 amino acid residues. CNP has been found in the central nervous system, in the cardiovascular system, in the digestive system, in the reproductive system, in the pulmonary system and widely distributed almost all over the body. It has been known as a local regular factor to exert biological actions in the way of paracrine and autocrine. The functions of CNP which are known at present are vasorelaxation, lowering blood pressure, anti-proliferation of vascular smooth muscle cells and cardiac fibroblasts, inhibition of hormonal secretions (LH, ANP, aldosterone), the activity of renin-angiotensin, and inhibition of basal motility of rabbit colon and oviduct, and development of ovarian follicle, reductions in jejunal fluid and electrolyte secretion to keep electrolyte homeostasis. Additionally, CNP was involved in the regulation of pathological and pathophysiological process of hypertension, chronic heart failure, chronic renal failure, atherosclerosis, and pulmonary proliferation.Our previous studies indicated that natriuretic peptide receptors were distributed in the gastric antral smooth muscles of rats, and CNP inhibited the gastric mobility in rats, guinea pigs and humans. Guo HS, et al, demonstrated that CNP could increasecalcium-actived potassium currents in gastric antral myocytes of guinea pigs, and the process was mediated by cGMP-PKG pathway. L-type calcium channel, one of the most important ion channels in gastric myocytes, was involved in the regulation of the gastric mobility. Up to now, the effect of CNP on L-type calcium channel and its mechanism has not been clear in gastric myocytes. So, the present study was designed to investigate the effect of CNP on L-type calcium channel in gastric antral myocytes of guinea pigs. The results were as follows:1. Under the conventional whole-cell configuration, PSS containing 2mmol·L-1 Ca2+ were superfused. The membrane potential was clamped at -80mV and ICa was elicited by step voltage command pulse from -40mV to +60mV for 400ms with a 10mV increment at 10 second intervals. I.Ca began to appear when the step voltage command pulse of -40mV was performed and reached its peak at 0mV with the average value of -124.02±56.34pA. The reversal potential was at about 45mV. Under the same configuration, PSS containing 5mmol·L-1 Ba2+ instead of 2mmol·L-1 Ca2+ were superfused and IBa was elicited by the same step voltage command pulse. At OmV the peak values about -587.91±95.55pA of IBa appeared and its reversal potential was at 50mV. Inactivation of IBa was slower than ICa and during the command pulse of 400ms, IBa didn't inactivate. Nicardipine(5μmol·L-1), the blockor of L-type calcium channel, inhibited IBa significantly, and the I/V relation was not altered compared with ICa.2. Under the conventional whole-cell configuration, the peak current of IBa was elicited with a voltage command pulse of OmV at 10 sec intervals. A time-course showed the peak responses of IBa at 0 mV (holding potential Vh=-80 mV) before and after administration with CNP (0.1μmol·L-1). IBa was immediately inhibited as soon as CNP was added, and the inhibitory effect came to stabilization within 2 minites. IBa recovered partially after being washed off with the normal control superfusing solution.3. Then under the same configuration, CNP significantly inhibited IBa, in a dose-dependant manner, and CNP inhibited IBa from control (100%) to 81.56%±2.48%, 73.64%±3.65%, 57.77%±4.93% at the concentrations of0.001μmol·L-1,0.01μmol·L-1 and 0.1μmol·L-1 at 0 mV, respectively.4. To determine whether the inhibition of IBa induced by CNP was mediated by cyclic GMP signal pathway, LY83583, an inhibitor of guanylate cyclase, was pretreated. After pretreatment with LY83583(0.1nmol·L-1) for 8-10 minutes, the inhibitory effect of CNP (0.1μmol·L-1) on IBa was significantly diminished, compared to the control.5. The effect of zaprinast, an inhibitor of cGMP sensitive phosphoesterase (PDE), was observed. In the presence of zaprinast (0.1μmol·L-1), the inhibitory effect of CNP (0.1μmol·L-1) on IBa was significantly potentiated.6. After pretreatment with KT5823 (0.1μmol L-1), a cGMP-dependent protein kinase (PKG) inhibitor, the inhibitory effect of CNP (0.1μmol·L-1) on IBa was significantly blocked.The results suggest that CNP significantly inhibited IBa in a dose-dependant manner in gastric antral myocytes of guinea pigs, and CNP-induced inhibition of IBa may be mediated by cGMP-PKG signal pathway in guinea pigs gastric antral myocytes.
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