IP3receptor-dependent Ca2+release modulates excitation-contraction coupling in rabbit ventricular myocytes

Abstract

Inositol 1,4,5-trisphosphate (IP3) receptor (IP3R)-dependent Ca2+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) Ca2+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 μM) produced a transient 21% increase in the frequency of Ca2+sparks ( P < 0.05). This increase was accompanied by a 13% decrease in spark amplitude ( P < 0.05) and a 7% decrease in SR Ca2+load ( P < 0.05) and was inhibited by IP3R antagonists 2-aminoethoxydiphenylborate (2-APB; 20 μM) and heparin (0.5 mg/ml). In intact myocytes endothelin-1 (100 nM) was used to stimulate IP3production and caused a 38% ( P < 0.05) increase in the amplitude of action potential-induced (0.5 Hz, field stimulation) Ca2+transients. This effect was abolished by the IP3R antagonist 2-APB (2 μM) or by using adenoviral expression of an IP3affinity trap that buffers cellular IP3. Together, these data suggest that in rabbit ventricular myocytes IP3R-dependent Ca2+release has positive inotropic effects on ECC by facilitating Ca2+release through ryanodine receptor clusters.