Increases in diastolic [Ca2+] can contribute to positive inotropy in guinea pig ventricular myocytes in the absence of changes in amplitudes of Ca2+ transients.
Increases in contraction amplitude following rest or in elevated extracellular Ca(2+) concentration ([Ca(2+)]) have been attributed to increased sarcoplasmic reticulum (SR) Ca(2+) stores and/or increased trigger Ca(2+). However, either manipulation also may elevate diastolic [Ca(2+)]. The objective of this study was to determine whether elevation of diastolic [Ca(2+)] could contribute to positive inotropy in isolated ventricular myocytes. Voltage-clamp experiments were conducted with high-resistance microelectrodes in isolated myocytes at 37 degrees C. Intracellular free [Ca(2+)] was measured with fura-2, and cell shortening was measured with an edge detector. SR Ca(2+) stores were assessed with 10 mM caffeine (0 mM Na(+), 0 mM Ca(2+)). 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 [Ca(2+)] but no change in Ca(2+) transient amplitudes. When extracellular [Ca(2+)] was elevated from 2.0 to 5.0 mM, resting intracellular [Ca(2+)] increased and resting cell length decreased. Amplitudes of contractions and L-type Ca(2+) current increased in elevated extracellular [Ca(2+)], although SR Ca(2+) stores, assessed by rapid application of caffeine, did not increase. Although Ca(2+) transient amplitude did not increase in 5.0 mM extracellular [Ca(2+)], diastolic [Ca(2+)] continued to increase with increasing extracellular [Ca(2+)]. These data suggest that increased diastolic [Ca(2+)] contributes to positive inotropy following rest or with increasing extracellular [Ca(2+)] in guinea pig ventricular myocytes.
Shutt
Robin H
RH
Dept. of Pharmacology, Sir Charles Tupper Medical Bldg., Dalhousie Univ., 5850 College St., Halifax, NS, Canada.
Ferrier
Gregory R
GR
Howlett
Susan E
SE
D000818:Animals; D002104:Cadmium / Q000494:pharmacology; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D003971:Diastole / Q000502:physiology; D004594:Electrophysiology; D006168:Guinea Pigs; D006352:Heart Ventricles / Q000166:cytology / Q000187:drug effects; D009200:Myocardial Contraction / Q000187:drug effects / Q000502:physiology*; D032383:Myocytes, Cardiac / Q000187:drug effects / Q000378:metabolism / Q000502:physiology*; D018408:Patch-Clamp Techniques; D012519:Sarcoplasmic Reticulum / Q000378:metabolism; D016276:Ventricular Function
D002104:Cadmium; D002110:Caffeine; D002118:Calcium