Insulin and aldosterone interaction on NA+ and K+ transport in cultured kidney cells (A6)

The effects of aldosterone (A) and insulin (I) on active Na+ transport [short circuit current (Isc)] and basolateral to apical K+ transport were evaluated in cultured epithelia derived from toad kidney (A6). Aldosterone plus insulin (A + I) stimulated Isc approximately 2 times the sum of the increments induced by A and I separately. Time courses of Isc stimulation by A + I in epithelia pretreated with A demonstrated that the synergism began about 80 min after the addition of insulin. The effect of A and I on basolateral to apical K+ transport was evaluated by apical K+ accumulation (flame photometry) in epithelia incubated for 24 h with K+-free medium initially placed on the apical side and various K+ concentrations on the basolateral side. At 4.5 mM basolateral K+, I had virtually no effect on basolateral to apical K+ transport, while both A and A + I produced significant increases (44% and 116%, respectively). Stimulation of basolateral to apical K+ transport by A + I was found to be concentration dependent. The EC50 for A stimulation (fixed I concentration) of K+ transport and Isc was 1-3 X 10(-8) M, which is quite similar to the Kd for nuclear binding of A previously reported. Our findings are consistant with the hypothesis that insulin, in addition to its own effects on Na+ transport, may cause an amplification of the mechanism of action of aldosterone on Na+ and K+ transport. The synergistic responses produced by A + I may represent an important hormonal interaction in regulation of transepithelial Na+ and K+ transport.