Distal colon from guinea pig was stimulated in vitro by aldosterone in Ussing chambers that allowed measurement of short-circuit current (Isc) and tissue conductance (Gt). The response to aldosterone was delayed by approximately 20 min and resulted in a negative Isc, consistent with K secretion. Approximately 1 h later the Isc began to increase and eventually became positive, consistent with subsequent stimulation of Na absorption. The Na-absorptive response could be inhibited by mucosal amiloride without altering the rate of K secretion. Similarly, K secretion could be inhibited by serosal bumetanide without altering Na absorption. In the presence of spironolactone, actinomycin D, or cycloheximide, aldosterone failed to stimulate both K secretion and Na absorption. A dose response to aldosterone provided an apparent Kd of 2.6 +/- 0.5 nM, consistent with a high-affinity receptor coupled to this secretory response. Stimulation by the K secretagogue epinephrine did not produce an additive increase in K secretion, suggesting that the same cell type responds to both aldosterone and epinephrine and that the protein induced by aldosterone was not one of the membrane proteins responsible for K secretion.
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