Calcium‐channel gating in frog skeletal muscle membrane: effect of temperature.

Abstract

SUMMARY1. Voltage-clamp experiments using the three micro-electrode method were performed to study the temperature dependence of the calcium current ICa in intact twitch skeletal muscle fibres of the frog. Contraction was blocked by recording in hypertonic sucrose solutions.2. For depolarizations smaller than 0 mV the decay of the transient, slow, inward current, recorded in the presence of external tetraethylammonium (TEA+) and by replacing Cl-for CH3SO3-, followed a complex time course. For larger depolarizations, after the initial inward current, there was a prominent, slow, outward current which showed two phases: after reaching a peak (time to peak 1-0 sec, peak amplitude 20-50 #sA/cm2 at 20 mV) it slowly declined to a steady level in about 2-3 see at 23 'C.3. The inward current was greatly reduced or abolished by the adding of2 mM-Cd2+ or by replacing external Ca2+ with Mg2+. The amplitude and time course of slow, outward currents were not obviously modified by replacing Ca2+ with Mg2+, having the two described phases. However, in the presence of Cd2+ the first transient phase of the outward current was not detected and only outward currents slowly increasing to a steady level were observed.4. Reliable ICa records were obtained by further blocking K+ outward currents by incubating the muscles in a K+-free TEA+-and Cs+-containing solution prior to experiments. Tubular space clamp was improved by recording ICa from small fibres with 20-30 jsm radius.5. The decay phase of ICa under a maintained depolarization in incubated muscles was fitted by a single exponential. The corresponding rate constant determined between 12 and 24 OC strongly depended on temperature, as expected for a gating process. The values for the activation energy and the corresponding Q10 (calculated for a 10-20 0C transition) were respectively: 17-5 + 1'0 kcal/mole and 2-9 + 0-2 at 0 mV, and 18-0 + 1-5 kcal/mole and 3 0 + 0-3 at -20 mV.6. The activation phase of Ica, analysed following the ma2h Hodgkin-Huxley kinetic model, showed a similar temperature dependence with a Q10 of 3-0 + 0 3. The

Kinetics of Inhibitory Feedback from Horizontal Cells to Photoreceptors: Implications for an Ephaptic Mechanism

“…Feedback slowed as the superfusate was cooled with a Q 10 for feedback time constants (measured at cone V m ϭ Ϫ20 mV) of ϳ3 (feedback current onset, 3.85 Ϯ 0.54, N ϭ 5; feedback current offset, 2.60 Ϯ 0.50; combined average, 3.22 Ϯ 0.40). However, the time constants for I Ca offset at termination of 100 ms step to Ϫ20 mV showed a similar Q 10 (2.70 Ϯ 0.95, N ϭ 9), consistent with reports on L-type channels in other preparations (Cota et al, 1983;Cavalié et al, 1985). Because Ca 2ϩ channel Q 10 values did not differ significantly from feedback Q 10 values (comparison with feedback offset, p ϭ 0.94, unpaired t test; comparison with feedback onset, p ϭ 0.42), these experiments did not help us to distinguish between chemical and ephaptic mechanisms.…”

Ionic basis of the different action potential configurations of single guinea‐pig atrial and ventricular myocytes.

“…They estimated current density to be 42,uA/cm2 in Tyrode solution containing 3-6 mM-CaCl2 at 35°C. This larger estimate of ica density is not inconsistent with our estimate of 5 /jA/cm2, considering the higher [Ca2+]0 used in their experiments and assuming a Q,0 ofapproximately 2 (Cota, Nicola Siri & Stefani, 1983).…”

Factors affecting the appearance of the hump charge movement component in frog cut twitch fibers.

“…The potential at which the OFF charge begins to exceed the ON charge varies from fiber to fiber, but in our hands is usually > -30 mV. One way to reduce the inward Ca current is to add Cd z÷ to the center pool solution (Cota et al, 1983;Donaldson and Beam, 1983).…”