The direct action of pineal melatonin on the renal system is supported
by our demonstration of 2-[I-125]iodomelatonin binding sites in the m
ale guinea pig kidney. Scatchard analyses and Hill coefficients reveal
ed a single type of binding site with an equilibrium dissociation cons
tant (Kd) of 22.3 +/- 1.6 pmol/l and a maximum binding density (Bmax)
of 0.99 +/- 0.03 fmol/mg protein (n = 7) at mid-light. There was no si
gnificant difference in the Kd and Bmax values between kidney tissues
collected at the middle of light and dark periods. The pharmacological
profile of these 2-[I-125]iodomelatonin binding sites indicated high
specificity for melatonin, 2-iodomelatonin and 6-chloromelatonin while
kinetic studies generated a Kd value of 28.4 +/- 7.3 pmol/l (n = 5) w
hich was comparable to that determined from Scatchard transformations.
Our results suggest that these binding sites are stable, reversible,
saturable, specific, and of high affinity. Regional distribution study
showed that specific binding of 2-[I-125]iodomelatonin was 8-fold hig
her in the cortical region than that in the medullary region. Studies
of subcellular distribution showed that 59.3% of binding sites were lo
calized in crude nuclear fractions followed by crude mitochondrial fra
ctions (22.3%) and crude microsomal fractions (18.3%) with no detectab
le binding in cytosolic fractions. Our present findings suggest the pr
esence of putative melatonin receptors in the guinea pig kidney, which
support the hypotheses of melatonin-regulated renin secretion togethe
r with renal excretory functions via melatonin receptors.