Enhanced L‐type Ca2+ channel current density in coronary smooth muscle of exercise‐trained pigs is compensated to limit myoplasmic free Ca2+ accumulation

Abstract

1 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. 2 Whole-cell voltage clamp experiments demonstrated enhanced VGCC density in smooth muscle cells freshly dispersed from coronary arteries of exercise-trained vs. sedentary animals. 3 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. 4 Inhibition of sarco-endoplasmic reticulum Ca²⁺-transporting ATPase (SERCA; 10 μm cyclopiazonic acid) and/or sarcolemmal Na⁺-Ca²⁺ exchange (low extracellular Na⁺) suggested neither mechanism compensated the enhanced VGCC in cells of exercise-trained animals. 5 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. 6 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.