Micro-electrode studies on the effects of exogenous cyclic adenosine monophosphate on active sodium transport in frog skin.

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Abstract
The electrical parameters of the sodium-transporting cells in frog skin of Rana angolensis were determined under control conditions by using the micro-electrode technique. The data were analysed in terms of an electrical model (Helman, 1979). The control intracellular voltages averaged -84.7 mV while the electromotive force of the inner barrier, E''1, averaged 103.9 mV. The major portion (82%) of the transcellular resistance was situated at the outer, apical, barrier. Exogenous cyclic AMP stimulated active sodium transport and the short-circuit current (Isc) increased by an average 88%. The change in Isc was mediated primarily by decreasing the resistance of the apical barrier (Ro) with little effect on the electromotive force or resistance (Ri) of the inner membranes. Isoprenaline increased the Isc by an average of 165%. The major effect of isoprenaline was to decrease the apical resistance by an average 77%. Forskolin (2.5 .mu.M) stimulated the Isc by an average of 138%. Amiloride would not completely reduce the Isc, but with the low concentrdation of 0.2 .mu.M-forskolin, the Isc was typically inhibited to values close to zero. The major effect of forskolin was also to reduce the resistance of the apical barrier, although it concurrently also caused the E''1 to decrease by about 13%. Theophylline increased the Isc by reducing the resistance of the apical barrier by an average 61%, with little or no effect on the other parameters. Theophylline augmented the effect of cyclic AMP. Our results are consistent with the theory that cyclic AMP is a second messenger in hormonal control of active sodium transport in frog skin.