Cyclic Nucleotide Metabolism in Pineal Homogenates

Adenylate cyclase (AC) in pineal paniculate fractions from rabbit, rat, cow, and the vole Microtus montanus was stimulated by L‐norepinephrine (NE) and L‐isopro‐terenol (ISO). NE stimulation of rabbit and bovine pineal AC was biphasic, with a plateau between 0.01 μM and 1.0 μM and additional stimulation by NE above 1.0 μM. Stimulation by different ISO concentrations gave a typical hyperbolic curve, and optimal stimulation by ISO exceeded that by NE. Melatonin decreased ISO and NE stimulation of AC 10–20%. Although α‐ad‐renergic agonists increase β‐agonist‐mediated adenosine‐3′, 5′‐cyclic monophosphate (cyclic AMP) accumulation in intact pinealocytes, similar amplification of AC stimulation was not seen with broken‐cell preparations. Most (60–70%) pineal guanylate cyclase (GC) was recovered in supernatant fractions after centrifugation of homogenates at 110,000 × g; this soluble GC was unaffected by potential agonists. Low concentrations (0.01–1 nM) of NE, ISO, and phenylephrine (PE) stimulated GC in impure and purified membrane fractions, but each inhibited at concentrations above 10 μM. All concentrations of ISO and NE inhibited GC in the presence of the a‐agonist PE. Melatonin alone did not affect particulate GC, but L‐ISO stimulation was not seen in the presence of equivalent concentrations of melatonin. The in vitro data are consistent with both α‐ and β‐receptor regulation of cyclic nucleotide metabolism in pinealocytes. Endogenous NE may differentially regulate cyclic AMP and guanosine‐3′, 5′‐cyclic monophosphate (cyclic GMP) in pineal; low NE concentrations that stimulate GC have only a slight effect on AC, but higher NE concentrations that inhibit GC maximally stimulate AC. Particulate GC and AC also were resolved by equilibrium centrifugation, to give several discrete peaks of enzyme activity. The results support the existence of several forms of AC and GC, which have different responses to adrenergic agonists.