Effect of methoxyindole administration on plasma cation levels in the rat.

Administration of methoxyindoles to rats in both the morning and the afternoon caused significant changes in circulating plasma cation levels. All the methoxyindoles except melatonin were without effect at doses below 100 micrograms/kg; the observed changes caused by doses of 100 micrograms/kg can, therefore, probably be best described as pharmacological rather than having physiological significance. Melatonin at all doses tested (25, 50, and 100 micrograms/kg) led to a decrease in plasma magnesium levels 3 h after administration in the morning, and this finding is discussed in the light of a possible feedback mechanism between melatonin and circulating magnesium. Melatonin administration in the afternoon was without effect on magnesium levels. Zinc levels were also decreased significantly 3 h after a morning dose of melatonin (50 and 100 micrograms/kg), and the possible functional significance of this observation is discussed. Most of the observed effects can be considered nonspecific and pharmacological, and further work is needed to fully elucidate the involvement, if any, of the pineal gland in regulation of cations.