Ocular penetration of beta-adrenergic blocking agents. An experimental study with atenolol, metoprolol, timolol and propranolol.

A comparison was made between the ocular penetration of topically applied 14C-atenolol, 14C-timolol, 14C-propranolol and 3H-metoprolol by means of liquid scintillation counting. Only one eye was treated, the fellow eye served as a control. Blood plasma levels were measured as well. We could detect a relationship between the ocular penetration and the degree of lipophilicity of the drugs used, as was to be expected. Drugs with a higher degree of lipophilicity penetrated more readily into the eye, whereas they also achieved higher blood plasma levels. Relatively high concentrations of atenolol were found in the nicitating membrane, thus reflecting its poor ocular penetration. The concentrations detected in the untreated eye were low and probably cannot explain the marked reduction in intraocular pressure observed after unilateral instillation of active beta-adrenergic blocking drugs, as for instance timolol. Our findings suggest that the ocular penetration of beta-adrenergic blocking agents on topical application only plays a minor part in their ocular hypotensive effect.

Penetration of atenolol in the rabbit eye.

The ocular penetration of topically applied [14C]-atenolol in the rabbit was determined by means of liquid scintillation counting. Only one eye was treated, the fellow eye serving as a control. Blood plasma levels were measured as well. The absolute amount of atenolol which penetrated the eye was very low, but a relatively high concentration was achieved in the tissues of the nictitating membrane. We could only detect an increase in the amount of atenolol with time in the aqueous humor. In all other ocular tissues, including iris and ciliary body, the atenolol level remained constant with time. Hardly any atenolol could be detected in the untreated eye and none in the blood plasma. These findings suggest that the ocular penetration of atenolol administered as an eye drop is very poor. Ocular penetration, therefore, hardly seems to play a part in the antiglaucomatous effect of atenolol.

The effects of morphine on the release of noradrenaline from the cat isolated nictitating membrane and the guinea-pig ileum myenteric plexus-longitudinal muscle preparation.

1. Electrical field stimulation of either the cat isolated nictitating membrane or the guinea-pig ileum myenteric plexus-longitudinal muscle preparation caused the release of noradrenaline into the bathing medium. 2. In the cat nictitating membrane, the output per pulse of noradrenaline was constant at frequencies of stimulation from 0.5 to 15 Hz. In the guinea-pig myenteric plexus preparation the output per pulse of noradrenaline increased as the frequency of stimulation was increased from 2 to 16 Hz. 3. Phenoxybenzamine (29.3 muM) caused a marked increase in the noradrenaline output from both the cat nictitating membrane and guinea-pig myenteric plexus preparations. 4. Morphine (0.13-8 muM) inhibited the contractions of the cat nictitating membrane caused by electrical stimulation. This effect was greater at low (1Hz) than at high (15Hz) frequencies of stimulat The site of action is at the nerve-smooth muscle junction. 5. The action of narcotic analgesic drugs on the cat nictitating membrane showed stereospecificity. Naloxone (0.1 muM) reversed the inhibition caused by normorphine (3.2 muM). 6. Morphine (3 muM) reduced the noradrenaline output from the cat nictitating membrane stimulated at 1 Hz but not at 15 Hz. At 1 Hz, the inhibition of noradrenaline output by normorphine (muM) was reversed by naloxone (0.25 muM). 7. Morphine (1.5 muM) did not alter the noradrenaline output from the guinea-pig myenteric plexus preparation stimulated at 2 or 16 Hz.

Adrenergic re-innervation of smooth muscle of nictitating membrane by preganglionic sympathetic fibres.

1. The preganglionic and post-ganglionic trunks of the cervical sympathetic nerve were joined in an end-to-end anastomosis after excision of the superior cervical ganglion in the cat.2. Seventy-five days after the anastomosis the diameter of the pupil was nearly normal and there was almost complete recovery of the prolapsed palpebra and of the nictitating membrane. The contraction of the nictitating membrane, induced by electrical stimulation, caudally to the point of anastomosis, showed that the smooth muscle of the nictitating membrane had been re-innervated.3. Neither hexamethonium nor nicotine had any marked effect on the contraction of the nictitating membrane. Severing the regenerated nerve trunk produced a degeneration contraction. These results are strong evidence that the denervated membranes were re-innervated by true cholinergic preganglionic fibres.4. Our pharmacological studies indicated that in the re-innervated preparations neuromuscular transmission was adrenergic in the sense that it was blocked by phentolamine and not by atropine. These results were confirmed by the histochemical-fluorescence studies which showed that the endings of the regenerated axons contained high concentrations of catecholamines.5. Electron microscopy showed that the regenerated terminals contained none of the small dense-core vesicles, considered to be typical of adrenergic nerve endings, but contained clear synaptic vesicles and an unusually great number of large granular vesicles.6. Our results suggest that the denervated nictitating membranes were re-innervated by cholinergic presynaptic sympathetic fibres that had been modified so that they could release catecholamines in addition to, or instead of, acetylcholine.