Modulation of intestinal chloride secretion at basolateral transport sites: opposing effects of cyclic adenosine monophosphate and phorbol ester.

BACKGROUND: Although intestinal Cl secretion is largely regulated by apical Cl channels, we have shown that net secretory capacity can be controlled at a basolateral site, the Na-K-2Cl cotransporter (NKCC). Phorbol myristate acetate (PMA) was found to inhibit both cyclic adenosine monophosphate (cAMP)-regulated Cl secretion and basolateral NKCC function in parallel but not apical Cl channels. Because inhibition of NKCC function could occur by reducing the number of membrane NKCC units, we examined the effect of PMA on cAMP-regulated NKCC function and number.
METHODS: NKCC function and number were assessed in the human intestinal line HT29cl.19A by bumetanide-sensitive uptake of rubidium 86 and by specific binding of 3H-bumetanide.
RESULTS: The cAMP agonist forskolin enhanced bumetanide-sensitive 86Rb uptake and doubled the number of NKCCs. PMA decreased both basal and cAMP-stimulated uptake in time- and dose-dependent fashion. In addition, PMA down-regulated basal NKCC number and abolished cAMP-induced NKCC recruitment.
CONCLUSIONS: PMA opposes the action of cAMP on NKCC function by reducing both basal numbers of NKCCs and cAMP-induced recruitment of NKCCs and not by reducing ion translocation per NKCC. These data further emphasize the potential for modulation of intestinal Cl secretion at basolateral sites.