Chronic parathyroid hormone administration reverses the antihypertensive effect of calcium loading in young spontaneously hypertensive rats.

To investigate the role of parathyroid hormone (PTH) in the development of hypertension, the long-term administration of human PTH (hPTH) was performed in 7-week-old male spontaneously hypertensive rats maintained on a standard calcium diet (1.1% calcium) or a high calcium diet (2.8% calcium). The hPTH was infused subcutaneously at 0.9 U/h over 2 weeks using a minipump. Two weeks after minipump implantation, rat PTH (rPTH) and 1,25-dihydroxy vitamin D (1,25-D) levels were measured and the pressor response to norepinephrine (NE) was examined. Calcium loading both attenuated the development of hypertension and reduced the pressor response to norepinephrine. Chronic hPTH administration accelerated the development of hypertension and increased the pressor response to norepinephrine in rats fed a high calcium diet, but did not affect either parameter in rats fed a standard calcium diet. The serum concentrations of rat PTH and 1,25-D were significantly lower in rats fed a high calcium diet than in those fed a standard calcium diet (rat PTH: 351 +/- 16 v 430 +/- 1 pmol/L; 1,25-D: 125 +/- 19 v 206 +/- 31 pg/mL). Chronic administration of hPTH increased the serum 1,25-D concentration in rats fed a high calcium diet (186 +/- 35 pg/mL), but did not affect that of rats fed a standard calcium diet (165 +/- 16 pg/mL). It is concluded that PTH prevented the antihypertensive effect of calcium loading in young SHR, perhaps by enhancing blood pressure reactivity through causing an increase of 1,25-D.