Bay K 8644 increases resting Ca2+ spark frequency in ferret ventricular myocytes independent of Ca influx: contrast with caffeine and ryanodine effects.
Bay K 8644, an L-type Ca2+ channel agonist, was shown previously to increase resting sarcoplasmic reticulum (SR) Ca2+ loss and convert post-rest potentiation to decay in dog and ferret ventricular muscle. Here, the effects of Bay K 8644 on local SR Ca2+ release events (Ca2+ sparks) were measured in isolated ferret ventricular myocytes, using laser scanning confocal microscopy and the fluorescent Ca2+ indicator fluo-3. The spark frequency under control conditions was fairly constant during 20 s of rest after interruption of electrical stimulation. Bay K 8644 (100 nmol/L) increased the spark frequency by 466+/-90% of control at constant SR Ca2+ load but did not change the spatial and temporal characteristics of individual sparks. The increase in spark frequency was maintained throughout the period of rest. The increase in Ca2+ spark frequency induced by Bay K 8644 was not affected by superfusion with Ca2+-free solution (with 10 mmol/L EGTA) but was suppressed by the addition of 10 micromol/L nifedipine (which by itself did not alter resting Ca2+ spark frequency). This suggests that the effect of Bay K 8644 on Ca2+ sparks is mediated by the sarcolemmal dihydropyridine receptor but is also independent of Ca2+ influx. Low concentrations of caffeine (0.5 mmol/L) increased both the average frequency and duration of sparks. Ryanodine (50 nmol/L) increased the spark frequency and also induced long-lasting Ca2+ signals. This may indicate long-lasting openings of SR Ca2+ release channels and a lack of local SR Ca2+ depletion. In lipid bilayers, Bay K 8644 had no effect on either single-channel current amplitude or open probability of the cardiac ryanodine receptor. It is concluded that Bay K 8644 activates SR Ca2+ release at rest, independent of Ca2+ influx and perhaps through a functional linkage between the sarcolemmal dihydropyridine receptor and the SR ryanodine receptor. In contrast, caffeine and ryanodine modulate Ca2+ sparks by a direct action on the SR Ca2+ release channels.
Satoh
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Department of Physiology and Cardiovascular Institute, Loyola University Chicago, Maywood, IL 60153, USA.
Katoh
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Velez
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Fill
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Bers
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D001498:3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethyl-5-nitro-4-(2-(trifluoromethyl)phenyl)-, Methyl ester / Q000378:metabolism / Q000494:pharmacology*; D000818:Animals; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D002120:Calcium Channel Agonists / Q000378:metabolism / Q000494:pharmacology*; D015220:Calcium Channels / Q000187:drug effects / Q000378:metabolism; D020746:Calcium Channels, L-Type; D005289:Ferrets; D006352:Heart Ventricles / Q000166:cytology* / Q000187:drug effects* / Q000378:metabolism; D015640:Ion Channel Gating / Q000187:drug effects; D007700:Kinetics; D008297:Male; D012433:Ryanodine / Q000494:pharmacology; D019837:Ryanodine Receptor Calcium Release Channel / Q000187:drug effects; D012508:Sarcolemma / Q000737:chemistry / Q000187:drug effects; D012519:Sarcoplasmic Reticulum / Q000737:chemistry / Q000187:drug effects; D013997:Time Factors
D002120:Calcium Channel Agonists; D015220:Calcium Channels; D020746:Calcium Channels, L-Type; D019837:Ryanodine Receptor Calcium Release Channel; D012433:Ryanodine; D002110:Caffeine; D001498:3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethyl-5-nitro-4-(2-(trifluoromethyl)phenyl)-, Methyl ester; D002118:Calcium