Ouabain-resistant hyperpolarization induced by insulin in aggregates of embryonic heart cells.

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Abstract
Spheroidal aggregates formed from trypsin-dissociated 14 day embryonic chicken hearts after 48 h of rotation on a gyratory shaker. Intracellularly recorded resting membrane potentials of aggregates bathed in 1.3 mM K+ balanced salt solution had a mean (.+-. SD) of 64 .+-. 4 mV. After a stable potential was achieved, addition of 1-100 nM Na bovine insulin caused a slow hyperpolarization of up to 19 mV after 4-5 min, followed, in some cases, by a further, more rapid, shift to a potential near EK [K equilibrium potential]. Equivalent hyperpolarizations were observed when insulin was added in the presence of 10 mM ouabain; enhanced Na+,K+ pump activity was not responsible for the change in membrane potential. The concentration of insulin that produced half-maximal hyperpolarization (2 nM) corresponded to the association constant of a high-affinity insulin receptor; apparently binding to this class of receptors led to the change in membrane potential. Steady-state current-voltage curves from current clamp experiments suggested that insulin produced an increase in slope conductance at potentials near rest by inducing an outward current with an apparent potential negative to -90 mV.