INOSITOL 1,4,5-TRISPHOSPHATE RECEPTOR IN HEART - EVIDENCE FOR ITS CONCENTRATION IN PURKINJE MYOCYTES OF THE CONDUCTION SYSTEM

Inositol 1,4,5-trisphosphate (IP3) is one of the second messengers cap able of releasing Ca2+ from sarcoplasmic reticulum/ER subcompartments. The mRNA encoding the intracellular IP3 receptor (Ca2+ channel) has b een detected in low amounts in the heart of various species by Norther n blot analysis. The myocardium, however, is a heterogeneous tissue co mposed of working myocytes and conduction system cells, i.e., myocytes specialized for the beat generation and stimulus propagation. In the present study, the cellular distribution of the heart IP3 receptor has been investigated. [H-3]IP3 binding experiments, Western blot analysi s and immunofluorescence, with anti-peptide antibodies specific for th e IP3 receptor, indicated that the majority of Purkinje myocytes (the ventricular conduction system) express much higher IP3 receptor levels than atrial and ventricular myocardium. Heterogeneous distribution of IP3 receptor immunoreactivity was detected both at the cellular and s ubcellular levels. In situ hybridization to a riboprobe generated from the brain type 1 IP3 receptor cDNA, showed increased accumulation of IP3 receptor mRNA in the heart conduction system. Evidence for IP3-sen sitive Ca2+ stores in Purkinje myocytes was obtained by double immunol abeling experiments for IP3 receptor and cardiac calsequestrin, the sa rcoplasmic reticulum intralumenal calcium binding protein. The present findings provide a molecular basis for the hypothesis that Ca2+ relea se from IP3-sensitive Ca2+ stores evoked by alpha1-adrenergic stimulat ion is responsible for the increase in automaticity of Purkinje myocyt es (del Balzo, U., M. R. Rosen, G. Malfatto, L. M. Kaplan, and S. F. S teinberg. 1990. Circ. Res. 67:1535-1551), and open new perspectives in the hormonal modulation of chronotropism, and generation of arrhythmi as.