ATTENUATION BY HYPOCALCEMIA OF PULSATILE GROWTH-HORMONE SECRETION IN CONSCIOUS MALE-RATS

The effect of hypocalcemia following parathyroidectomy (PTX) on growth hormone (GH) secretion was investigated in unrestrained, unanesthetized male rats bearing chronically implanted indwelling cannulae. During a 6 h period, starting at about 1000 h, control rats with a serum Ca value of 8.11 .+-. 0.38 mg/dl (mean .+-. SEM [standard error of the mean]) 2 wk after sham-operation showed secretory bursts of GH similar to those observed in conscious intact rats. Under hypocalcemia of 4.88 .+-. 0.32 mg/dl 2 wk after PTX, GH secretory episodes were completely suppressed throughout the study. Plasma prolactin (PRL) levels were also decreased in PTX rats as compared with those of sham-operated rats. Daily food intake and body wt gain as well as serum T4 [thyroxine] levels in PTX rats were not different from those of sham-operated and intact rats. Pituitary GH content of PTX rats was significantly lower than that of sham-operated and control rats. Pulsatile GH secretion was partially restored in PTX rats by raising serum Ca to 8.43 .+-. 0.27 mg/dl through feeding with high Ca diet containing 7% Ca. Immediately after i.v. injection of antisomatostatin sheep serum, pulsatile GH surges recovered in PTX rats despite hypocalcemia of 4.48 .+-. 0.74 mg/dl. The mean plasma 6 h GH levels were significantly higher than those of normal sheep-serum-treated PTX rats (P < 0.001). Apparently the episodic release of GH is suppressed in hypocalcemic rats after PTX, at least partially via circulating endogenous somatostatin.