Dopamine enhances the phosphaturic effect of PTH during acute respiratory alkalosis

Abstract

The phosphaturic response to parathyroid hormone (PTH) is blunted during acute respiratory alkalosis. The objective of the present study was to determine the effect of dopamine on the blunted phosphaturic response to PTH during acute respiratory alkalosis. The phosphaturic response to PTH was determined in thyroparathyroldectomized (TPTX) normocapnic and respiratory alkalotic rats in the absence and presence of the infusion of exogenous dopamine (25 μg/kg/min) or of 3,4-dihydroxyphenylalanine (l-DOPA, 250 μg/kg/min) to increase endogenous dopamine synthesis. In normocapnic rats, PTH infusion (33 U/kg plus 1 U/kg/min) significantly increased the fractional excretion of phosphate (FEpi), from 1.5% ± 0.5% to 28.4% ± 4.0%, (ΔFEpi 26.9% ± 4.1%, n = 11, P < .05); in respiratory alkalotic rats, the increase was from 0.4% ± 0.1% to 11.4% ± 1.7% (ΔFEpi 11.0% ± 1.8%, n = 13, P < .05). However, the phosphaturic response to PTH was attenuated in respiratory alkalotic rats (ΔFEpi 26.9% ± 4.1% vs 11.0% ± 1.9%, P < .05). In normocapnic rats, in the presence of dopamine or l-DOPA infusions, PTH infusion significantly increased the FEpi, from 6.1% ± 2.3% to 33.4% ± 8.0% (ΔFEpi 27.3% ± 7.0%, n = 5) and from 3.2% ± 0.6% to 32.5% ± 3.3% (ΔFEpi 29.3% ± 3.2%, n = 7), respectively. In respiratory alkalotic rats, in the presence of dopamine infusion, PTH significantly increased the FEpi, from 0.6% ± 0.2% to 19.3% ± 3.3% (ΔFEpi 18.7% ± 3.3%, n = 6); in the presence of l-DOPA infusion it increased from 1.0% ± 0.3% to 20.5% ± 2.8% (ΔFEpi 19.5% ± 2.9%, n = 8, P < .05 as compared with PTH alone). Thus the phosphaturic effect of PTH that was attenuated in respiratory alkalotic rats was enhanced by stimulation of endogenous dopamine synthesis by the infusion of l-DOPA.