Calmodulin activated adenylyl cyclase in ciliary processes: additivity of calcium and cyclic adenosine monophosphate signals on intraocular pressure response of the rabbit eye.
OBJECTIVE
The regulation of adenylyl cyclase by multiple signal systems was investigated by biochemical studies of the enzyme in ciliary processes and by in vivo intraocular pressure responses in the rabbit eye.


METHODS
Adenylyl cyclase enzyme activity was determined by radiometric assay using alpha-32 P-ATP as substrate. Drugs were administered to the rabbit eye by intravitreal injection or topical application, and intraocular pressure was measured by pneumatonography.


RESULTS
Adenylyl cyclase activity in the membrane/particulate fraction of the rabbit ciliary process was activated by calmodulin in the presence of Ca2+, Co2+, or Mn2+, and inhibited by the calmodulin antagonist calmidazolium. The activity was additive to stimulations of adenylyl cyclase by the activating G-protein (Gs) via isoproterenol or vasoactive intestinal peptide receptors, and by forskolin. The biochemical findings were supported by in vivo correlation experiments with intravitreal injection of MnCl2, and by topical treatment with the Ca(2+)-mobilizing alpha 1-adrenergic agonist phenylephrine and with agents affecting cyclic adenosine monophosphate levels (forskolin, isobutylmethylxanthine). The intraocular pressure response was augmented by combining threshold doses of phenylephrine with threshold doses of either forskolin or isobutylmethylxanthine. The maximal and most prolonged response (a decrease in intraocular pressure of 4 mm Hg for up to 8 hr) was obtained by combination treatment with all three agents at doses that produced no significant effect by themselves.


CONCLUSIONS
These findings provide a biochemical mechanism for ocular hypotensive drugs having both a Ca(2+)-mobilizing activity as well as increasing cyclic adenosine monophosphate levels by receptor-coupled stimulation of adenylyl cyclase.
Department of Ophthalmology, Mount Sinai School of Medicine, New York, NY 10029.
T W
TW
Mittag
A
A
Tormay
T
T
Taniguchi
M
M
Ortega
D000262:Adenylyl Cyclases / Q000378:metabolism*; D000818:Animals; D001082:Aqueous Humor / Q000378:metabolism; D002118:Calcium / Q000494:pharmacology*; D002147:Calmodulin / Q000494:pharmacology*; D002412:Cations / Q000494:pharmacology; D002924:Ciliary Body / Q000187:drug effects / Q000201:enzymology*; D000242:Cyclic AMP / Q000494:pharmacology*; D004357:Drug Synergism; D004789:Enzyme Activation; D005136:Eye Proteins / Q000378:metabolism; D007429:Intraocular Pressure / Q000187:drug effects*; D008565:Membrane Proteins / Q000187:drug effects; D011817:Rabbits; D014065:Tonometry, Ocular
D002147:Calmodulin; D002412:Cations; D005136:Eye Proteins; D008565:Membrane Proteins; D000242:Cyclic AMP; D000262:Adenylyl Cyclases; D002118:Calcium