DISTINCT REGULATION OF NA- SECRETION BY ARGININE-VASOPRESSIN IN THE AMPHIBIAN CELL-LINE A6( REABSORPTION AND CL)

The neurohypophysial peptide arginine vasopressin (AVP) increases Naabsorption across A6 epithelia. In addition to the positive natriferic response, AVP increases net basolateral to apical Cl- flux. The time course of activation of electrogenic ion transport in A6 epithelia was examined by measuring transepithelial short-circuit current (I(SC)). Basolateral application of AVP (0.1 U/ml) or forskolin (10 muM) affect s I(SC) in a biphasic manner. Shortly after addition of AVP, an early (transient) phase is observed in which I(SC) is rapidly stimulated, re aching a peak value at 1.4 +/- 0.1 min. A subsequent decrease in curre nt is interrupted by a slower, late phase in which I(SC) reaches a pea k 23 +/- 3 min after addition of AVP. The late increase in I(SC) is su stained over the remainder of the 40-min period of observation. The ti me course of I(SC) stimulation by forskolin is qualitatively similar. Replacement of external Cl- by aspartate lowers baseline transport nea rly 40%, strongly blunts the early phase of I(SC) stimulation, and ret ains the late increase. Addition of amiloride (10 muM) to the apical b ath before AVP or forskolin stimulation of I(SC) eliminates the late i ncrease of I(SC). Steady-state amiloride-insensitive I(SC) activated u nder these conditions was sensitive to apical application of the Cl- c hannel blockers 5-nitro-2-(3-phenylpropylamino)-benzoate (20 muM) and niflumic acid (100 muM). 4, 4'-Diisothiocyanostilbene-2,2'-disulfonic acid (1 mM) was not an effective inhibitor of this current. Basolatera l bumetanide (100 muM) inhibited baseline I(SC) and reduced both the p eak transient and steady-state amiloride-insensitive I(SC). These data suggest that the early phase of I(SC) activation across A6 monolayers by AVP or forskolin represents rapid regulation of Cl- secretion. The late phase represents both steady-state Cl- secretion and gradual upr egulation of Na+ reabsorption.