UNUSUAL RESPONSES OF NOCTURNAL PINEAL MELATONIN SYNTHESIS AND SECRETION TO SWIMMING - ATTEMPTS TO DEFINE MECHANISMS

The effect of swimming at night on rat pineal melatonin synthesis was compared with that of light exposure at night. Rats were forced to swi m at 0030 hr (lights out at 2000 hr) and sacrificed by decapitation 15 and 30 min later, immediately after swimming. Other groups of animals were exposed to white light (650 muW/cm2) for 15 and 30 min at same t ime. Swimming caused a rapid and highly significant drop in the melato nin content in the pineal gland; however, the activity of N-acetyltran sferase (NAT), the supposed rate limiting enzyme in the melatonin prod uction, was not changed. Despite the drop in pineal melatonin levels, serum concentrations of the indole remained elevated in the rats that swam. In contrast, melatonin levels in the pineal and serum of light e xposed rats fell precipitously, accompanied by a significant suppressi on of NAT activity. Since we anticipated that the strenuous exercise a ssociated with swimming may induce release of artrial natriuretic pept ide (ANP) from the heart, which in turn could cause the release of pin eal melatonin, in a second study we injected physiological saline intr avenously to stretch the cardiac muscle and release ANP. Three millili ters of normal saline was injected during the day into the jugular vei n of anesthetized rats that were pretreated with isoproterenol to stim ulate pineal melatonin production. Animals were killed 15 min after th e saline injection, and pineal NAT activity and pineal melatonin level s were measured. The saline injections caused no alteration in the ele vated levels of either NAT or melatonin. These data suggest that the d isparity in pineal NAT activity (which was high) and pineal melatonin (which was low), in animals swum at night, may not be caused by ANP wh ich is released during strenuous exercise such as swimming.