Dual effect of secretin on nephron filtration and proximal reabsorption depending on the route of administration.

Secretin is a vasoactive peptide capable of acting on transmembrane volume fluxes. We measured nephron filtration (SNGFR) and resorption during secretin microinjection (MIJ) into the tubular lumen or microperfusion (MP) into peritubular capillaries. In 24 rat nephrons, SNGFR, measured by collections from the distal tubule, rose from 25+/- 4 to 61+/-8 nl/min during MIJ of saline containing secretin 10(-9) M into the last convolution of the proximal tubule (LP). Percent and absolute resorptions rose from 70 to 90% and from 20+/-4 to 56+/-8 nl/min, respectively. During MIJ of secretin, 3 x 10(-)(9) M into the first convolution of the proximal tubule, SNGFR, measured at LP, rose from 32+/-4 to 61+/-8 nl/min, percent and absolute reabsorptions from 52+/-4 to 78+/-3% and from 16+/-2 to 50+/-7 nl/min, respectively (n = 30). During MP of secretin, 1.5x10(-9) M, SNGFR fell from 39+/-6 to 15+/-4, resorption from 19+/-4 to 9+/-2 nl/min, while percent resorption rose from 43+/-6 to 59+/-5% (n = 15). While all MIJ and MP changes were significant (P<0.001), paired pre- versus post-MIJ and MP values were not. Secretin is a powerful vasoconstrictor when perfused into peritubular capillary blood, unlike systemic and intra-arterial injections. When injected into the tubular lumen, it up-regulates SNGFR and increases reabsorption directly.