The effects of adrenergic agonists on intraocular pressure and on adenylyl cyclase activity of ciliary processes in pigmented rabbits.

The effects of beta-adrenergic agonists isoprenaline, fenoterol and clenbuterol on the activity of adenylyl cyclase from ciliary processes and on intraocular pressure were examined in pigmented rabbits. Isoprenaline, fenoterol and clenbuterol stimulated adenylyl cyclase activity in vitro, but clenbuterol behaved as a partial agonist. Preincubation of ciliary processes with any of these three drugs led to the heterologous desensitization of adenylyl cyclase to the stimulatory effects of beta-adrenergic agonists or vasoactive intestinal peptide (VIP). This desensitization was dose-dependent and was expressed mainly as a decrease of the highest effects of stimulatory drugs. The exact mechanism of this phenomenon is not yet known. After topical administration, all three tested beta-adrenergic agonists decreased intraocular pressure with approximately the same intensity. The relationship between ocular hypotensive effects of beta-adrenergic agonists and their effects on adenylyl cyclase of ciliary processes is discussed. It is concluded that ocular hypotensive effects of adrenergic agonists and other drugs stimulating adenylyl cyclase cannot be explained simply by stimulation or desensitization of adenylyl cyclase of ciliary processes.

Repeated topical administration of fenoterol in rabbit reverses its initial ocular hypotensive effect and decreases sensitivity of adenylyl cyclase in ciliary processes to stimulatory agents.

PURPOSE: The effects of repeated topical administration of the selective beta 2-adrenergic agonist fenoterol on the intraocular pressure and on the adenylyl cyclase activity in ciliary processes in rabbit were examined in order to detect their possible causal relationship. METHODS: Intraocular pressure was measured by pneumatonometry. Adenylyl cyclase activity in homogenates of ciliary processes was assayed ex vivo by measurement of conversion of 32P-alpha-ATP to 32P-cyclic AMP. RESULTS: A single topical dose of 1% solution of fenoterol elicited a clear-cut decrease of the intraocular pressure lasting for several h. Repeated administration of fenoterol for 2-5 days led to a significant increase of intraocular pressure, observable from the second to the fifth day. The stimulation of adenylyl cyclase activity ex vivo by isoproterenol, vasoactive intestinal polypeptide or forskolin was significantly decreased on the fifth day (24 h after the administration of the last dose of fenoterol). CONCLUSIONS: Our data showed that repeated topical administration of the selective beta 2-adrenergic agonist increased intraocular pressure and desensitized adenylyl cyclase in ciliary processes; if these two effects are related then they would support the idea of direct relationship of decreased cAMP production in ciliary processes to the increase of intraocular pressure, and vice versa. However, conclusive evidence of this suggestion and of its possible significance in another animal species or man would require further study.

Effects of drugs acting on adrenergic and adenosine receptors on the intraocular pressure and the activity of adenylyl cyclase in ciliary processes and their sensitivity to pertussis toxin.

The effects of the selective alpha2-adrenergic agonist p-aminoclonidine, the nonselective adrenergic agonist epinephrine, the selective beta2-adrenergic agonist fenoterol and the adenosine A1 agonist R-PIA on intraocular pressure were studied in control and pertussis toxin-pretreated rabbits. Pretreatment of rabbits with pertussis toxin decreased the ocular hypotensive effects of p-aminoclonidine and epinephrine, did not influence the same effects of fenoterol or R-PIA and markedly potentiated the initial ocular hypertensive effects of epinephrine and R-PIA. As far as the action on adenylyl cyclase in ciliary processes is concerned, isoproterenol stimulated its activity in control rabbits and epinephrine exerted dual, i.e. stimulatory and inhibitory effects on the activity of this enzyme. The data obtained with epinephrine and p-aminoclonidine confirm the view that their ocular hypotensive effects are associated with their inhibitory action on adenylyl cyclase and contradict the opinion that the hypotensive action of adrenergic drugs depends on adenylyl cyclase activation.

Effects of pertussis toxin on intraocular pressure and adenylate cyclase activity of ciliary processes in rabbits.

The effects of pertussis toxin on the intraocular pressure (IOP) lowering effect of clonidine and isoproterenol as well as on the inhibitory effects of clonidine and neuropeptide Y on adenylate cyclase activity of ciliary processes were studied in albino rabbits. I.v. administered pertussis toxin elicited transient changes in IOP which, however, returned to control values during 2-3 days. In the following days the IOP lowering effect of the alpha 2-adrenergic agonist clonidine was abolished and that of the nonselective beta-adrenergic agonist isoproterenol was attenuated. At the same time, the inhibitory effects of clonidine and neuropeptide Y on basal as well as stimulated adenylate cyclase activities in homogenates of ciliary processes were grossly diminished. The effects of pertussis toxin on the IOP lowering action of adrenergic agonists and on the inhibitory action of clonidine and neuropeptide Y on adenylate cyclase activity were ascribed to an impairment of the function of a G protein in ciliary processes, probably G(i) protein. It is suggested that the decrease of IOP induced by clonidine is due to inhibition of adenylate cyclase.

Inhibitory effects of neuropeptide Y on adenylate cyclase of rabbit ciliary processes.

The inhibitory effect of neuropeptide Y (NPY) was studied on the adenylate cyclase (AC) activity in homogenates of rabbit ciliary processes and compared with that of the alpha 2-adrenergic agonist clonidine (CLN). NPY inhibited basal AC activity as well as AC activity stimulated by isoproterenol (ISO), vasoactive intestinal polypeptide (VIP) or forskolin (FSK). The extent of this inhibition corresponded well to the inhibition elicited by CLN. The inhibitory effects of NPY and CLN appeared to be nonadditive. AC activity stimulated by ISO was considerably more sensitive to the effects of either NPY or CLN than basal, VIP- or FSK-stimulated AC activity. It was inferred that NPY inhibitory effects were mediated by the activation of NPY receptors coupled negatively to the catalytic unit of AC via the inhibitory Gi protein. Moreover, involvement of NPY in physiological modulation of AC activity in ciliary processes and in the regulation of aqueous humor formation and intraocular pressure is suggested.

Inhibitory effects of clonidine and dopamine on adenylate cyclase of rabbit ciliary processes.

The inhibitory effects of the alpha 2-adrenergic agonist clonidine and that of dopamine were studied on the adenylate cyclase activity in homogenates of ciliary processes. Clonidine inhibited in a dose-dependent manner basal adenylate cyclase activity as well as that stimulated by isoproterenol or forskolin. However, the adenylate cyclase activity stimulated by isoproterenol was sensitive to at least one order lower inhibitory concentrations of clonidine than basal or forskolin-stimulated adenylate cyclase. Dopamine inhibited adenylate cyclase stimulated by isoproterenol considerably less potently than clonidine. The slope of the dopamine dose-response curve was, however, similar to that of the dose-response curve of clonidine. The inhibitory effects of clonidine and dopamine were antagonized by an alpha 2-adrenergic antagonist, yohimbine, in a manner suggesting a competitive nature of this interaction. On the contrary, the inhibitory effects of neither clonidine nor dopamine were prevented by an alpha 1-adrenergic antagonist, prazosin. In addition, the effect of dopamine was not antagonized by the D2-antagonist, tiapride. Taken together, these results strongly indicate that both clonidine and dopamine exert their inhibitory effects by the stimulation of alpha 2-adrenergic receptors. Accordingly, they provide experimental evidence that both basal and drug-stimulated adenylate cyclase activity of ciliary processes can be inhibited via stimulation of alpha 2-adrenergic receptors. The substantially higher sensitivity of isoproterenol-stimulated than basal or forskolin stimulated adenylate cyclase to alpha 2-adrenergic inhibition seems to be a unique feature of this enzyme of ciliary processes. It is suggested that this may reflect an involvement of alpha 2-adrenergic receptors in the physiological feedback mechanism preventing the over-stimulation of adenylate cyclase of ciliary processes during excessive adrenergic drive.

Effect of different albumin media on the lipid-mobilizing action of sympathicotropic substances in vitro.

The authors studied the effect of adrenotropic substances on lipolysis in rat epididymal adipose tissue in albumin medium in vitro. On using albumins of different origin (human, bovine), the pD2 values for catecholamines differed by more than one order, in correlation to the type of albumin used. The isopropylnorsynephrine pD2 values did not differ. The addition of ascorbic acid (100 microng/ml) raised the catecholamine pD2 values and completely equalized the pD2 values found in both media. The pD2 values for the synephrine derivative did not alter. The propranolol pA2 values were not negatively affected by the addition of ascorbic acid. Ascorbic acid also produced a mild increase in the maximum lipid-mobilizing values obtained with any of the given substances in either medium. It was concluded in the discussion that catecholamines are oxidized at different rates in different albumin media and that this oxidation can be inhibited by adding ascorbic acid. Ascorbic acid likewise mildly stimulates the maximum lipid-mobilizing effect. The authors recommend the addition of ascorbic acid to albumin medium as a regular component for the study of adrenergic lipid mobilization.