Effects of increase in plasma calcium concentration on renal handling of NaCl and NaHCO3.

Recollection micropuncture experiments were carried out in thyroparathyroidectomized volume-expanded rats to examine the effects of CaCl2 infusion on the renal and nephronal segmental handling of chloride and bicarbonate. In group 1A, a 0.23 mM increase in plasma calcium concentration [delta(Ca)P] reduced urinary total CO2 (tCO2) excretion from 401 +/- 90 to 166 +/- 43 nmol X min-1 X g kidney wt-1 (P less than 0.05), whereas tCO2 filtered load was slightly diminished from 34,086 +/- 3,627 to 28,904 +/- 2,496 nmol X min-1 X g kidney wt-1 (NS). In group 1B [delta(Ca)P, 0.73 mM], whole kidney filtered loads were significantly lowered, as was urinary tCO2 excretion; however, urinary excretion of sodium, chloride, and water remained constant. Calcium infusion inhibited the proximal reabsorption of chloride 25% and water 16%; however, calcium infusion caused the end-proximal tCO2 concentration to significantly decrease so that the absolute and fractional tCO2 reabsorption remained constant. In group 2 [delta(Ca)P, 0.43 mM], whole kidney filtered load was unchanged for chloride and water but decreased for bicarbonate; urinary tCO2 excretion was reduced, whereas chloride and water excretion increased. In this group, early distal micropunctures evidenced that superficial single-nephron filtered loads were significantly reduced during calcium infusion; early distal chloride delivery was enhanced from 348 +/- 32 to 441 +/- 36 pmol X min-1 X g kidney wt-1 (P less than 0.05), whereas tCO2 delivery decreased from 47 +/- 5 to 38 +/- 4 pmol X min-1 X g kidney wt-1 (P less than 0.05). In group 3 of time control animals, whole kidney and early distal data were unchanged during second period. In group 4, H+ secretion in the collecting duct, as assessed by analyzing the relationship between urine-minus-blood PCO2 and urinary bicarbonate concentration in maximally alkaline urine, was not modified during CaCl2 infusion [delta(Ca)P, 0.79 mM]. We conclude that increase in plasma calcium concentration inhibits proximal NaCl and water reabsorption, whereas it stimulates the bicarbonate transport relative to that of chloride, leading to an enhanced proximal and renal bicarbonate-to-chloride reabsorptive ratio that could generate metabolic alkalosis; and decreases urinary bicarbonate excretion by also lowering the bicarbonate filtered load.