Mechanism of fluid secretion in isolated shark renal proximal tubules.

Renal proximal tubules from the glomerular Squalus acanthias were studied for evidence of fluid secretion by closing one end of isolated tubules and leaving the other end open so that secreted fluid could be collected. When tubules were bathed in shark Ringer, fluid secretion rate was 27.6 +/- 3.9 (SE) pl X min-1 X mm-1 (21 tubules). Dibutyryl cAMP stimulated fluid secretion 50% (P less than 0.02, n = 14), furosemide inhibited fluid secretion 50% (P less than 0.01, n = 6), and metabolic inhibitors blocked fluid secretion nearly 100%. Secreted fluid was slightly hyperosmotic to peritubular bath (P less than 0.01, n = 7) but Na, Cl, S, K, and Ca concentrations were not significantly different from bath concentrations (wavelength-dispersive spectroscopy, electron probe analysis). cAMP had no effect on secreted fluid composition, and in some tubules cAMP did not stimulate fluid secretion. In conjunction with previous data we propose that spontaneous fluid secretion is driven by secretion of NaCl. However, finding the mechanism of NaCl and fluid secretion in glomerular renal tubules offers new perspectives of some previously inexplicable phenomena in the renal physiology of fish.