Ca2+ entry-independent effects of L-type Ca2+ channel modulators on Ca2+ sparks in ventricular myocytes.
During the cardiac action potential, Ca(2+) entry through dyhidropyridine receptor L-type Ca(2+) channels (DHPRs) activates ryanodine receptors (RyRs) Ca(2+)-release channels, resulting in massive Ca(2+) mobilization from the sarcoplasmic reticulum (SR). This global Ca(2+) release arises from spatiotemporal summation of many localized elementary Ca(2+)-release events, Ca(2+) sparks. We tested whether DHPRs modulate Ca(2+)sparks in a Ca(2+) entry-independent manner. Negative modulation by DHPR of RyRs via physical interactions is accepted in resting skeletal muscle but remains controversial in the heart. Ca(2+) sparks were studied in cat cardiac myocytes permeabilized with saponin or internally perfused via a patch pipette. Bathing and pipette solutions contained low Ca(2+) (100 nM). Under these conditions, Ca(2+) sparks were detected with a stable frequency of 3-5 sparks.s(-1).100 microm(-1). The DHPR blockers nifedipine, nimodipine, FS-2, and calciseptine decreased spark frequency, whereas the DHPR agonists Bay-K8644 and FPL-64176 increased it. None of these agents altered the spatiotemporal characteristics of Ca(2+) sparks. The DHPR modulators were also without effect on SR Ca(2+) load (caffeine-induced Ca(2+) transients) or sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) activity (Ca(2+) loading rates of isolated SR microsomes) and did not change cardiac RyR channel gating (planar lipid bilayer experiments). In summary, DHPR modulators affected spark frequency in the absence of DHPR-mediated Ca(2+) entry. This action could not be attributed to a direct action of DHPR modulators on SERCA or RyRs. Our results suggest that the activity of RyR Ca(2+)-release units in ventricular myocytes is modulated by Ca(2+) entry-independent conformational changes in neighboring DHPRs.
Copello
Julio A
JA
Dept. of Pharmacology, Southern Illinois University School of Medicine, Springfield, IL 62794-9629, USA. jcopello@siumed.edu
Zima
Aleksey V
AV
Diaz-Sylvester
Paula L
PL
Fill
Michael
M
Blatter
Lothar A
LA
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002121:Calcium Channel Blockers / Q000494:pharmacology; D020746:Calcium Channels, L-Type / Q000378:metabolism*; D020013:Calcium Signaling / Q000502:physiology; D002415:Cats; D002462:Cell Membrane / Q000378:metabolism; D005260:Female; D006352:Heart Ventricles / Q000166:cytology / Q000378:metabolism*; D008297:Male; D032383:Myocytes, Cardiac / Q000187:drug effects / Q000378:metabolism*; D019837:Ryanodine Receptor Calcium Release Channel / Q000378:metabolism; D053498:Sarcoplasmic Reticulum Calcium-Transporting ATPases / Q000378:metabolism
D002121:Calcium Channel Blockers; D020746:Calcium Channels, L-Type; D019837:Ryanodine Receptor Calcium Release Channel; D053498:Sarcoplasmic Reticulum Calcium-Transporting ATPases; D002118:Calcium