The effect of supplementary administration of recombinant bovine somatotrophin (rbST) on the renal tubular handling of sodium in crossbred 87.5% Holstein cattle housed in normal shade (NS) or mist-fan cooled (MF) barns was evaluated. The cows were injected with 500 mg rbST at three different stages of lactation. The MF barn housed cows showed a slightly decreased ambient temperature and temperature humidity index, but an increased relative humidity. Rectal temperature and respiration rates were significantly lower in cooled cows. The rbST treated cows, housed in NS or MF barns, showed markedly increased milk yields, total body water, extracellular fluid and plasma volume levels, along with a reduced rate of urine flow and urinary excretion of sodium, potassium and chloride ions and osmolar clearance, in all three stages of lactation. Renal tubular sodium and water reabsorption were increased after rbST administration without any alteration in the renal hemodynamics. Lithium clearance data suggested that the site of response is in the proximal nephron segment, which may be mediated via increases in the plasma levels of aldosterone and IGF-1, but not vasopressin, during rbST administration.
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