Hypothalamic involvement in stress-induced hypocalcemia in rats

Lateral hypothalamic injection of GABA(A) antagonist induces gastric vagus-mediated hypocalcemia in the rat

The involvement of lateral hypothalamic area (LHA) neurons in the regulation of blood calcium homeostasis was investigated in unanesthetized rats. The microinjection of the gamma-aminobutyric acid A receptor antagonist bicuculline methiodide (BM, 4-40 ng x 0.5 microl(-1) x 5 min(-1)) into the LHA decreased the blood concentration of ionized calcium. Total serum calcium also decreased after the BM injection. This hypocalcemic effect was eliminated by a bilateral vagotomy of the gastric branches. An intravenous injection of atropine methyl bromide (a muscarinic antagonist), nadolol (a beta-adrenergic blocker), or ranitidine (a histamine H2 blocker) suppressed the BM-induced hypocalcemia, whereas phenoxybenzamine (an alpha-adrenergic blocker) proved to be ineffective. Although the intra-LHA injection of BM increased the serum gastrin, which is known to have a hypocalcemic effect, neither secretin nor somatostatin (gastrin-release inhibitors) blocked the hypocalcemic response. These results suggest that the hypocalcemia observed after the excitation of LHA neurons was mediated by muscarinic, beta-adrenergic, and histamine H2 receptors through the gastric vagus.

Hypothalamic stimulation induces vagally mediated hypocalcemia in the rat

To elucidate hypothalamic involvement in blood calcium homeostasis, the effects of unilateral electrical stimulation (0.1 mA, 0.5 ms, 30 Hz, 60 min) of the lateral hypothalamic area (LHA), the paraventricular nucleus (PVN), and the ventromedial nucleus of the hypothalamus (VMH) on the blood concentration of ionized calcium were examined in the anesthetized rats. LHA stimulation induced a sustained decrease (0.05-0.07 mM fall) in the blood calcium level during the period of 60 to 150 min (end of the measurements) after stimulation. In contrast, PVN stimulation elicited a transient hypocalcemia (0.07 mM decrease) 60 min after stimulation. The hypocalcemic effects of LHA and PVN stimulation were eliminated by vagotomy of the gastric branches and the thyroid/parathyroid branches, respectively. VMH stimulation, using the same parameters, did not induce any significant change in blood calcium. The results suggest that the LHA and the PVN have a hypocalcemic function that is mediated, at least in part, by the vagus nerve innervating the stomach and the thyroid/parathyroid glands, respectively.

Ventromedial hypothalamus mediates stress-induced hypocalcemia via the gastric vagus in rats

A hypothalamo-vagal mechanism of immobilization (IMB) stress-induced hypocalcemia was investigated in rats. Bilateral lesions in the ventromedial nucleus of the hypothalamus (VMH), but not those of the lateral hypothalamic area (LHA) or the paraventricular nucleus (PVN), eliminated the calcium-lowering effect of IMB. None of these lesions, however, affected the basal levels of the blood calcium. An electrical stimulation of the VMH induced a significant decrease in the blood calcium level (0.07 mM fall) 60 min after stimulation. The hypocalcemic response was eliminated by a vagotomy of the gastric branches but not by that of the thyroid/parathyroid branches. These results suggest that the VMH mediates IMB-induced hypocalcemia through its influence on the gastric vagus.