cAMP increases the basolateral Cl- -conductance in the isolated perfused medullary thick ascending limb of Henle's loop of the mouse.

The effect of cAMP on transepithelial and transmembrane potential differences and resistances was examined in isolated in vitro perfused mouse medullary thick ascending limbs of Henle's loop (mTAL). The effects of furosemide and barium were tested. Stimulation of NaCl transport by ADH 10(-9) + dbcAMP 4 X 10(-4) + forskolin 10(-6) mol X l-1 (paired experiments) resulted in: a) an increase in transepithelial potential difference, referenced to the grounded bath, from +6.7 +/- 0.3 mV to +12.0 +/- 0.4 mV (n = 47); b) a decrease in transepithelial resistance from 25 +/- 1 omega cm2 to 20 +/- 1 omega cm2 (n = 47); c) a depolarization of the basolateral membrane by 12 mV and of the apical membrane by 7 mV (n = 36); d) a decrease in the fractional resistance of the basolateral membrane from 0.27 +/- 0.05 to 0.15 +/- 0.06 (n = 12). Furosemide (10(-4) mol X l-1) abolished the active transepithelial transport potential and hyperpolarized the basolateral membrane potential to values which were similar in both control and cAMP treated mTAL segments. Barium increased the transepithelial resistance and depolarized PDb1 to similar values in both functional states. An increase in the fractional conductance of the basolateral membrane was also seen, if, prior to the cAMP treatment, the luminal Na+2Cl-K+ cotransport was inhibited by furosemide. Thus, we propose that stimulation of active NaCl reabsorption in the mTAL segment of the mouse by ADH, mediated via cAMP, increases primarily the basolateral chloride conductance.