Cardiac binding of [³H]inositol 1,4,5-trisphosphate following chronic stimulation of cyclic AMP signalling in guinea pigs

Binding of [³H]inositol 1,4,5-trisphosphate ([³H]IP₃) to guinea pig heart has been characterised, localised and the effect of the in vivo desensitisation of cardiac β-adrenoceptor-cyclic AMP signalling examined. Quantitative autoradiography showed highest levels of [³H]IP₃ binding associated with coronary blood vessels and with the endothelial cells of the aorta and the mitral and tricuspid valves. Moderate levels of [³H]IP₃ bound to the atrioventricular conducting system, cardiac valves and aortic smooth muscle. Lower levels of [³H]IP₃ binding were detected in atrial and ventricular myocardium. Although the phosphoinositide signalling pathway does not contribute greatly to normal cardiac function, there is evidence of an increased importance in situations of compromised excitation-contraction such as occurs in cardiac failure or with, β-adrenoceptor desensitisation. We examined whether chronic in vivo stimulation of β-adrenoceptor-adenylate cyclase signalling affected cardiac binding of [³H]IP₃. Infusion of guinea pigs with isoprenaline (400 μg kg^-1 h^-1, 7 days) tended to reduce [³H]IP₃ binding in myocardium although not significantly (P > 0.05, n = 4). These data indicate that [³H]IP₃ binding has a heterogeneous distribution in guinea pig heart with highest levels of binding discretely localised to endothelial cells. Desensitisation of β-adrenoceptor-cyclic AMP signalling in heart did not lead to upregulation of [³H]IP₃ binding.