Cell pairs isolated from adult guinea pig and rat hearts: effects of [Ca2+]i on nexal membrane resistance

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Abstract
Cell pairs isolated from adult rat and guinea pig ventricles were used to study the resistance of the nexal membrane,r n. Each cell of a cell pair was connected to a voltage-clamp circuit to obtain simultaneous whole-cell, tight-seal recordings. With this technique,r n was determined under experimental conditions aimed at primarily modifying [Ca2+]i. Moderate changes in [Ca2+]i (produced by trains of depolarizing voltage-clamp pulses activating the slow inward current, or alterations in [Ca2+]o from 0.5 to 10 mM), resulted in no change inr n for normally coupled cell pairs (r n=5 MΩ), but small and reversible changes in slightly uncoupled preparations (r n≥50 MΩ). Large increases inr n developed with substantial elevations in [Ca2+]i (secondary to [Na+]o-withdrawal, exporsure to strophanthidin in conjunction withi si, or Ca2+-dialysis). Increases inr n brought about via elevation in [Ca2+]i always were accompanied by cell shortening consistent with a sustained contracture. The current-voltage relationship of the nexal membrane was ohmic regardless of whetherr n was low (control) or elevated (after increasing [Ca2+]i).