Site of restoration of the effect of PTH by propranolol in respiratory alkalosis

Dopamine enhances the phosphaturic effect of PTH during acute respiratory alkalosis

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 thyroparathyroidectomized (TPTX) normocapnic and respiratory alkalotic rats in the absence and presence of the infusion of exogenous dopamine (25 microg/kg/min) or of 3,4-dihydroxyphenylalanine (L-DOPA, 250 microg/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 (FE(Pi)), from 1.5%+/-0.5% to 28.4%+/-4.0%, (deltaFE(Pi) 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% (deltaFE(Pi) 11.0%+/-1.8%, n = 13, P<.05). However, the phosphaturic response to PTH was attenuated in respiratory alkalotic rats (deltaFE(Pi) 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 FE(Pi) from 6.1%+/-2.3% to 33.4%+/-8.0% (deltaFE(Pi) 27.3%+/-7.0%, n = 5) and from 3.2%+/-0.6% to 32.5%+/-3.3% (deltaFE(Pi) 29.3%+/-3.2%, n = 7), respectively. In respiratory alkalotic rats, in the presence of dopamine infusion, PTH significantly increased the FE(Pi), from 0.6%+/-0.2% to 19.3%+/-3.3% (deltaFE(Pi) 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% (deltaFE(Pi) 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.

Propranolol blocks the hypophosphaturia of acute respiratory alkalosis in human subjects

Respiratory alkalosis (RA) is seen in diverse clinical conditions including tissue hypoxia, malignancy, neurologic disorders, febrile states, pregnancy, and hepatic failure. Acute RA causes hypophosphaturia in rats, and this effect on renal phosphate handling is reversed by beta-adrenoreceptor antagonism. The objective of the present study was to determine the effect of acute RA on phosphate excretion in human patients in the absence and presence of beta-adrenoreceptor antagonism with propranolol. Twelve normal volunteers, 6 women and 6 men, were studied in two phases, once with placebo and once with intravenous infusion of propranolol. In both groups, 30-minute renal clearances were taken during normoventilation (NV) and during acute RA induced by voluntary hyperventilation. Acute RA produced a significant decrease in plasma phosphate (PPi) in the absence (deltaPPi = -0.16 +/- 0.03 mmol/L) and the presence (deltaPPi = -0.16 +/- 0.05 mmol/L) of propranolol. In the placebo group, fractional excretion of phosphate (FEPi) decreased from 24.1% +/- 3.4% in NV to 19.2% +/- 2.6% in RA. This was associated with a significant decrease in parathyroid hormone (PPTH), from 3.38 +/- 0.28 pmol/L in NV to 2.54 +/- 0.30 pmol/L in RA. In the propranolol group, FEPi did not change significantly, from 19.1% +/- 2.7% in NV to 18.7% +/- 3.0% in RA. This also occurred in the face of a decrease in PPTH, from 4.39 +/- 0.53 pmol/L in NV to 2.78 +/- 0.33 pmol/L in RA. Thus propranolol selectively changes the response of FEPi to acute RA while leaving the PPi and PPTH responses unaltered. This suggests that beta-adrenoreceptors play a role in the regulation of the response of renal phosphate handling during acute RA and that this role involves a direct tubular effect on phosphate reabsorption, independent of filtered load and hormonal status. We conclude that beta-adrenoreceptor antagonism blunts the hypophosphaturic effect of acute respiratory alkalosis in human subjects.