Renal electrolyte handling, acid-base status, and urinary acidification in Bufo marinus

In hydrated toads reabsorption of 92% of filtered bicarbonate produces a urine of pH at 22 degrees C. Average blood-to-urine pH gradient is 0.8-0.9 U under these conditions but may reach 3-4 U. Net acid excretion rate in hydrated animals is approximately 30 mu eq . h-1 . kg-1. Flow-independent acid excretion is a negative nonlinear function of urinary pH and passes from positive to negative values in the range of normal urinary pH due to rapid rise in [HCO3-] and slower decline in [NH4+]. [H2PO4-] accounts for only 10-20% of buffer-bound acid excreted. Sulfate loading in Cl-restricted toads is without effect on blood pH but produces significant reduction in blood [Cl-]/[Na+] and in urinary pH, and increased urinary [K+] and net acid secretion rates relative to controls. These renal responses are diminished or absent during sulfate loading in Cl-rich toads. In both groups [Cl-] shows significant positive correlation with pH in sulfate-containing urines. These results are discussed in the context of the nonhomeostatic model of urinary acidification developed in mammals.