The correlation between the increase in slow outward current and in contraction induced by caffeine, ryanodine, and rapid cooling in voltage-clamped frog muscle fibers.
The effects of caffeine, ryanodine, and rapid cooling were tested on the depolarization-induced contraction and the apamin-insensitive slow outward current (Iso) of voltage-clamped (double mannitol gap) single frog muscle fibers. Subthreshold caffeine concentrations (0.5-2 mM) induced a monotonic increase in contractile and Iso amplitude. Whatever the concentration, the increase in contraction was roughly twice the one in current. Similar results were obtained upon rapid cooling (20-4 degrees C) in the presence of 0.5 mM caffeine. In the absence of external Na+ (choline-substituted) 10(-5) M ryanodine induced a delayed increase (approximately 30 min) in contraction and in current, shortly before the development of a drastic and irreversible contracture. Here again, the increase in contraction was twice that in current. In the presence of 5 mM tetraethylammonium (TEA) and (or) 25 nM charybdotoxin, 2 mM caffeine still induced a strong facilitating effect on contraction but the parallel increase in current was strongly reduced. The linear relationship between the increase in current and contractile amplitude has a slope approximately 0.5 (whatever the drug used to increase contractility); it is approximately 0.1 in the presence of TEA and (or) charybdotoxin. In conclusion, provided the changes in contractile amplitude are caused by parallel changes in depolarization-induced sarcoplasmic reticulum Ca2+ release, about 50% of the apamin-insensitive Iso is controlled by internal Ca2+ release. The main part of this current corresponds to the TEA- and charybdotoxin-sensitive component of Iso.
Nasri-Sebdani
M
M
Laboratory of General Physiology, URA C.N.R.S. 290, Faculty of Sciences, Poitiers, France.
Traoré
F
F
Mouzou
A
A
Cognard
C
C
Potreau
D
D
Raymond
G
G
D000818:Animals; D001030:Apamin / Q000494:pharmacology; D002110:Caffeine / Q000494:pharmacology*; D002118:Calcium / Q000378:metabolism; D018999:Charybdotoxin; D003080:Cold Temperature; D066298:In Vitro Techniques; D007473:Ion Channels / Q000187:drug effects*; D008564:Membrane Potentials / Q000187:drug effects; D009119:Muscle Contraction / Q000187:drug effects; D009132:Muscles / Q000187:drug effects* / Q000502:physiology; D011895:Rana ridibunda; D012433:Ryanodine / Q000494:pharmacology*; D012519:Sarcoplasmic Reticulum / Q000187:drug effects / Q000378:metabolism; D012604:Scorpion Venoms / Q000494:pharmacology; D012996:Solutions; D013757:Tetraethylammonium Compounds / Q000494:pharmacology
D007473:Ion Channels; D012604:Scorpion Venoms; D012996:Solutions; D013757:Tetraethylammonium Compounds; D018999:Charybdotoxin; D012433:Ryanodine; D001030:Apamin; D002110:Caffeine; D002118:Calcium