B-type natriuretic peptide (BNP) attenuates the L-type calcium current and regulates ventricular myocyte function

A fundamental question in physiology is how hormones regulate the functioning of a cell or organ. It was therefore the aim of this study to investigate the effect(s) of BNP-32 on calcium handling by ventricular myocytes obtained from the rat left ventricle. We specifically tested the hypothesis that BNP-32 decreased the L-type calcium current (ICa,L). Perforated patch clamp technique was used to record ICa,L and action potential (AP) in voltage and current clamp mode, respectively. Myocyte shortening was measured using a photodiode array edge-detection system and intracellular calcium transients were measured by fluorescence photometry. Western blotting was used to determine the relative change in the expression of proteins. At the concentrations tested, BNP-32 significantly decreased cell shortening in a dose-dependent manner; increased the phase II slope of the AP by 53.0%; increased the APD50 by 16.9%; reduced the ICa,L amplitude with a 22.9% decrease in the peak amplitude and reduced Ca2+-dependent inactivation; increased the V1/2 activation of the L-type calcium channel by 51.1% and decreased V1/2 inactivation by 31.8%; and, intracellular calcium transient amplitude was significantly decreased by 32.0%, whereas the time to peak amplitude and T1/2 were both significantly increased by 38.7% and 89.4% respectively. Sarcoplasmic reticulum Ca2+-ATPase (SERCA2a) protein expression was reduced by BNP-32. These data suggest that BNP-32 regulates ventricular myocyte function by attenuating ICa,L, altering the AP and reducing SERCA2a activity and/or expression. This study suggests a novel constitutive mechanism for the autocrine action of BNP on the L-type calcium channel in ventricular myocytes.