Temperature dependence of embryonic cardiac gap junction conductance and channel kinetics

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Abstract
We have investigated the effects of temperature on the conductance and voltage-dependent kinetics of cardiac gap junction channels between pairs of seven-day embryonic chick ventricle myocytes over the range of 14–26°C. Records of junctional conductance (G j) and steady-state unit junctional channel activity were made using the whole-cell double patch-clamp technique while the bath temperature was steadily changed at a rate of about 4°C/min. The decrease inG j upon cooling was biphasic with a distinct break at 21°C. In 12 cell pairs,Q 10 was 2.2 from 26 to 21°C, while between 21 and 14°C it was 6.5. The meanG j at 22°C (G j22) was 3.0±2.1 nS, ranging in different preparations from 0.24 to 6.4 nS. At room temperature, embryonic cardiac gap junctions contain channels with conductance states near 240, 200, 160, 120, 80 and 40 pS. In the present study, we demonstrate that cooling decreases the frequency of channel openings at all conductance levels, and at temperatures below 20°C shifts the prevalence of openings from higher to lower conductance states: all 240 pS openings disappear below 20°C; 200 pS openings are suppressed at 17°C; below 16°C 160 and 120 pS events disappear and only 80 and 40 pS states are seen. Temperature also affected the voltage-dependent kinetics of the channels. Application of a 6 sec, 80 mV voltage step across the junction (V j80) caused a biexponential decay in junctional conductance. Decay was faster at lower temperatures, whereas the rate of recovery ofG j after returning toV j0 was slowed. Cooling reduced the fast decay time constant, increased both recovery time constants, and decreased the magnitude of GitGj decay, thus leaving a 10–16% larger residual conductance (G ss/G init,±80 mVV j) at 18 than at 22°C. From these results we propose that embryonic chick cardiac gap junctions contain at least two classes of channels with different conductances and temperature sensitivities.