Enhanced vascular reactivity to various vasoconstrictor agents following pinealectomy in the rat: role of melatonin.

The mesenteric vascular bed preparation of control and pinealectomized (PX) male Wistar rats was used to examine vascular reactivity to two concentrations each of norepinephrine, serotonin, angiotensin, and potassium. Vasoconstrictor responses to 50- and 100-ng injections of norepinephrine and 0.5- and 1.0-microgram injections of serotonin were 30-40% higher in preparations from PX rats. Reponses to 100 ng but not to 50 ng of angiotensin were also significantly higher in preparations from PX rats. Responses to 1.5- and to 3.0-mg injections of potassium did not differ significantly in either case. In vivo injection of 20 microgram of melatonin 3 h prior to dissection of the preparation, or in vivo perfusion of 20 ng melatonin per millilitre of buffer completely reversed the increased vascular response to all vasoconstrictor agents tested in the PX preparations, but had little effect in control preparations. Also observed in PX rats was a significant increase in blood pressure, serum sodium, and increased body and heart weight. Arterial wall sodium was also elevated in PX rats. These changes may be relevant to the increased vascular reactivity of PX rats. The increased vascular responsiveness of PX rats may be specific for agents that stimulate calcium release from intracellular stores (norepinephrine, angiotensin) rather than those that stimulate calcium influx from extracellular fluid (potassium). Melatonin lack may be the cause of the vascular changes in the PX rats as both in vivo and in vitro it lowered the vasoconstrictor effects of the agents tested, but only in PX rats; it had no significant effect in the control rats.