Insulin-Mediated H + and NH 4 + Excretion in the Urinary Bladder of Bufo marinus 1

Abstract

This study was done to determine if insulin mediates H⁺ and NH₄ ⁺ excretion in the urinary bladder of Bufo marinus. Acidosis was induced by gavaging with 10 ml of 120 mM NH₄Cl 3× daily for 2 days. Hemibladders were mounted between Lucite chambers. Insulin (porcine) was added to the serosal solution of the experimental bladder (10² mU/ml). After a 15-min equilibration the flux was measured for 2 hr. H⁺ excretion was measured from change in pH of the mucosal fluid and the NH₄ ⁺ measured colorimetrically. The excretion was normalized for weight of bladder and reported in units of nanomoles (100 mg bladder)^-1 (min)^-1. Plasma insulin was determined by radioimmunoassay and glucose by the glucose oxidase method. In 14 control bladders H⁺ excretion was 8.75 ± 1.28 and experimental was 16.35 ± 2.50 (P < 0.025), while NH4 excretion in control bladder was 3.29 ± 0.95 and experimental was 6.58 ± 1.89 (P < 0.01). This response was absent when the insulin was heat inactivated (P > 0.2 and P > 0.3 respectively). Plasma insulin-like levels in 10 normal toads was 0.57 ± 0.16 ngm/ml and in acidotic toads 1.25 ± 0.16 ng/ml (P < 0.025). Plasma glucose levels in 10 normal toads were 22.0 ± 3.5 mg/dl and in 12 acidotic toads 17.8 ± 0.75 mg/dl (P < 0.025). We conclude that plasma insulin is increased in acidosis and that insulin stimulates excretion of H⁺ and NH₄ ⁺ in the toad urinary bladder.