Increases in diastolic [Ca2+] can contribute to positive inotropy in guinea pig ventricular myocytes in the absence of changes in amplitudes of Ca2+ transients

Abstract

Increases in contraction amplitude following rest or in elevated extracellular Ca2+ concentration ([Ca2+]) have been attributed to increased sarcoplasmic reticulum (SR) Ca2+ stores and/or increased trigger Ca2+. However, either manipulation also may elevate diastolic [Ca2+]. The objective of this study was to determine whether elevation of diastolic [Ca2+] could contribute to positive inotropy in isolated ventricular myocytes. Voltage-clamp experiments were conducted with high-resistance microelectrodes in isolated myocytes at 37°C. Intracellular free [Ca2+] was measured with fura-2, and cell shortening was measured with an edge detector. SR Ca2+ stores were assessed with 10 mM caffeine (0 mM Na+, 0 mM Ca2+). Following a period of rest, cells were activated with trains of pulses, which generated contractions of increasing amplitude, called positive staircases. Positive staircases were accompanied by increasing diastolic [Ca2+] but no change in Ca2+ transient amplitudes. When extracellular [Ca2+] was elevated from 2.0 to 5.0 mM, resting intracellular [Ca2+] increased and resting cell length decreased. Amplitudes of contractions and L-type Ca2+ current increased in elevated extracellular [Ca2+], although SR Ca2+ stores, assessed by rapid application of caffeine, did not increase. Although Ca2+ transient amplitude did not increase in 5.0 mM extracellular [Ca2+], diastolic [Ca2+] continued to increase with increasing extracellular [Ca2+]. These data suggest that increased diastolic [Ca2+] contributes to positive inotropy following rest or with increasing extracellular [Ca2+] in guinea pig ventricular myocytes.