IP3 receptor-dependent Ca release modulates excitation-contraction coupling in rabbit ventricular myocytes

Domeier TL, Zima AV, Maxwell JT, Huke S, Mignery GA, Blatter LA. IP3 receptor-dependent Ca release modulates excitation-contraction coupling in rabbit ventricular myocytes. Am J Physiol Heart Circ Physiol 294: H596–H604, 2008. First published November 30, 2007; doi:10.1152/ajpheart.01155.2007.— Inositol 1,4,5-trisphosphate (IP3) receptor (IP3R)-dependent Ca signaling exerts positive inotropic, but also arrhythmogenic, effects on excitation-contraction coupling (ECC) in the atrial myocardium. The role of IP3R-dependent sarcoplasmic reticulum (SR) Ca release in ECC in the ventricular myocardium remains controversial. Here we investigated the role of this signaling pathway during ECC in isolated rabbit ventricular myocytes. Immunoblotting of proteins from ventricular myocytes showed expression of both type 2 and type 3 IP3R at levels 3.5-fold less than in atrial myocytes. In permeabilized myocytes, direct application of IP3 (10 M) produced a transient 21% increase in the frequency of Ca sparks (P 0.05). This increase was accompanied by a 13% decrease in spark amplitude (P 0.05) and a 7% decrease in SR Ca load (P 0.05) and was inhibited by IP3R antagonists 2-aminoethoxydiphenylborate (2-APB; 20 M) and heparin (0.5 mg/ml). In intact myocytes endothelin-1 (100 nM) was used to stimulate IP3 production and caused a 38% (P 0.05) increase in the amplitude of action potential-induced (0.5 Hz, field stimulation) Ca transients. This effect was abolished by the IP3R antagonist 2-APB (2 M) or by using adenoviral expression of an IP3 affinity trap that buffers cellular IP3. Together, these data suggest that in rabbit ventricular myocytes IP3R-dependent Ca release has positive inotropic effects on ECC by facilitating Ca release through ryanodine receptor clusters.