Enhanced L-type Ca2+ channel current density in coronary smooth muscle of exercise-trained pigs is compensated to limit myoplasmic free Ca2+ accumulation.
We hypothesized that enhanced voltage-gated Ca2+ channel current (VGCC) density in coronary smooth muscle cells of exercise-trained miniature Yucatan pigs is compensated by other cellular Ca2+ regulatory mechanisms to limit net myoplasmic free Ca2+ accumulation. Whole-cell voltage clamp experiments demonstrated enhanced VGCC density in smooth muscle cells freshly dispersed from coronary arteries of exercise-trained vs. sedentary animals. In separate experiments using fura-2 microfluorometry, we measured depolarization-induced (80 mM KCl) accumulation of myoplasmic free Ba2+ and free Ca2+. Both maximal rate and net accumulation of free Ba2+ in response to membrane depolarization were increased in smooth muscle cells isolated from exercise-trained pigs, consistent with an increased VGCC density. Depolarization also produced an enhanced maximal rate of free Ca2+ accumulation in cells of exercise-trained pigs; however, net accumulation of free Ca2+ was not significantly increased suggesting enhanced Ca2+ influx was compensated to limit net free Ca2+ accumulation. Inhibition of sarco-endoplasmic reticulum Ca2+-transporting ATPase (SERCA; 10 microM cyclopiazonic acid) and/or sarcolemmal Na+-Ca2+ exchange (low extracellular Na+) suggested neither mechanism compensated the enhanced VGCC in cells of exercise-trained animals. Local Ca2+-dependent inactivation of VGCC, assessed by buffering myoplasmic Ca2+ with EGTA in the pipette and using Ca2+ and Ba2+ as charge carriers, was not different between cells of sedentary and exercise-trained animals. Our findings indicate that increased VGCC density is compensated by other cellular Ca2+ regulatory mechanisms to limit net myoplasmic free Ca2+ accumulation in smooth muscle cells of exercise-trained animals. Further, SERCA, Na+-Ca2+ exchange and local Ca2+-dependent inactivation of VGCC do not appear to function as compensatory mechanisms. Additional potential compensatory mechanisms include Ca2+ extrusion via plasma membrane Ca2+-ATPase, mitochondrial uptake, myoplasmic Ca2+-binding proteins and other sources of VGCC inactivation.
Heaps
C L
CL
Dalton Cardiovascular Research Center and Department of Physiology, University of Missouri, Columbia, MO 65211, USA. heaps@missouri.edu
Bowles
D K
DK
Sturek
M
M
Laughlin
M H
MH
Parker
J L
JL
D000818:Animals; D001464:Barium / Q000493:pharmacokinetics; D002118:Calcium / Q000378:metabolism* / Q000502:physiology; D020746:Calcium Channels, L-Type / Q000502:physiology*; D003331:Coronary Vessels / Q000502:physiology*; D004553:Electric Conductivity; D005260:Female; D005456:Fluorescent Dyes; D016257:Fura-2; D009131:Muscle, Smooth, Vascular / Q000502:physiology*; D009210:Myofibrils / Q000378:metabolism; D010805:Physical Conditioning, Animal; D012519:Sarcoplasmic Reticulum / Q000378:metabolism; D019831:Sodium-Calcium Exchanger / Q000502:physiology; D013552:Swine; D013556:Swine, Miniature
D020746:Calcium Channels, L-Type; D005456:Fluorescent Dyes; D019831:Sodium-Calcium Exchanger; D001464:Barium; D002118:Calcium; D016257:Fura-2