Differences in specific activity of calcium in blood and urine of dogs

Abstract

Following intravenous administration of radioactive calcium, no differences were observed between blood and urine calcium specific activities even at early time periods. Following intraperitoneal administration of similar isotope solutions, however, a considerable discrepancy was noted between the specific activity in the two compartments, the urine specific activity remaining lower than that of the blood for as long as 5 hr. This same phenomenon was apparent after intravenous administration of radioactive calcium if a solution were injected intraperitoneally at the time of isotope administration. Blood/urine specific activity ratios exceeding unity were associated with periods of low calcium excretion. Adrenergic drugs, injected intravenously, were found to mimic intraperitoneal injections of isotope solutions. The response to intraperitoneal injections could not be blocked by atropine, but was absent in the presence of Dibenamine. It was concluded that an intraperitoneal injection had altered renal hemodynamics through a neural response which appeared to be sympathetically mediated. No evidence was found for a nonhomogeneous distribution of exogenous labeled calcium among the plasma calcium fractions. It is suggested that kidney tissue calcium is the source of the disproportionate amount of stable calcium appearing in the urine.