FAILURE OF WATER IMMERSION TO INFLUENCE PARATHYROID-HORMONE SECRETION AND RENAL PHOSPHATE HANDLING IN NORMAL MAN

  • 1 January 1976
    • journal article
    • research article
    • Vol. 87  (2) , 218-226
Abstract
Previous studies from this laboratory have demonstrated that the redistribution of blood volume and concomitant relative central hypervolemia induced by water immersion to the neck (NI) results in a significant natriuuresis which is quantitatively identical to that induced by the acute administration of 2 l of saline. Since the central hypervolemia induced by NI occurs without concomitant alterations in serum ionized calcium concentration (Ca2+), the NI model was utilized to assess the role of volume in the regulation of PTH [parathyroid hormone] secretion in man. Normal subjects (7) were studied following 11 h of dehydration on 2 occasions, control and NI. The conditions of seated posture and time of day were identical. Blood for Ca2+ and PTH was obtained at 30-min intervals for 6 h. NI resulted in a significant increase in urinary Na excretion from a prestudy value of 78 .+-. 12 (standard error of the mean) to 222 .+-. 20 .mu.eq [equivalent]/min (P < 0.001). Concomitantly, Ca2+ remained constant, ranging between 4.57 to 4.71 mg percent. Despite the volume-induced natriuresis, PTH was not altered throughout 5 h of NI, ranging from 36 .+-. 7 to 45 .+-. 5 .mu.l-eq/ml. Phosphate excretion remained constant. These data indicate that central volume expansion does not alter PTH in normal man when the variables of ionized Ca2+, posture, and time of day are controlled. The current demonstration of the absence of phosphaturia during immersion despite the probability that the distal delivery of phosphate was enhanced, permits consideration of the possibility that the concept of a distal tubular reabsorptive site for phosphate may be applicable to man.