Adrenergic receptors in bovine retinal microvessels: presence of alpha 2- and beta- but not alpha 1-receptors

Characterization and ontogeny of PGE2 and PGF2 alpha receptors on the retinal vasculature of the pig

The vasoconstrictor effects of PGE2 and PGF2 alpha are less pronounced on retinal vessels of the newborn than of the adult pig. We tested the hypothesis that the decreased vasomotor response to these prostaglandins might be due to relatively fewer receptors and/or different receptor subtypes (in the case of PGE2) on retinal vessels of the newborn animal. Binding studies using [3H]PGE2 and [3H]PGF2 alpha revealed that PGE2 (EP) and PGF2 alpha (FP) receptor densities in retinal microvessel membrane preparations from newborn animals were approximately 25% of those found in vessels from the adult. The Kd for PGF2 alpha did not differ; however, the Kd for PGE2 was less in newborn than in adult vessels. Competition binding studies using AH 6809 (EP1 antagonist), butaprost (EP2 agonist), M/+B 28,767 (EP3 agonist), and AH 23848B (EP4 antagonist) suggested that the retinal vessels of the newborn contained approximately equal number of EP1 and EP2 receptor subtypes whereas the main receptor subtype in the adult vessels was EP1. In addition, PGE2 and butaprost produced comparable increases in adenosine 3',5'-cyclic monophosphate synthesis in newborn and adult vessels. PGE2, 17-phenyl trinor PGE2 (EP1 agonist) and PGF2 alpha caused a 2.5 to 3-fold greater increase in inositol 1,4,5-triphosphate (IP3) formation in adult than in newborn preparations. It is concluded that fewer PGF2 alpha receptors and an associated decrease in receptor-coupled IP3 formation in the retinal vessels of the newborn could lead to weaker vasoconstrictor effects of PGF2 alpha on retinal vessels of the newborn than of adult pigs; fewer EP1 receptors (associated with vasoconstriction) and a relatively greater proportion of EP2 receptors (associated with vasodilation) might be responsible for the reduced retinal vasoconstrictor effects of PGE2 in the newborn.

Effects of beta-adrenergic blockade on vasodepressor reaction in patients with vasodepressor syncope

The mechanism of vasodepressor reaction induced by head-up tilt test in patients with vasodepressor syncope is not clearly understood. We hypothesized that an abnormal alteration of beta-adrenergic transmission could be involved in these patients. We measured plasma catecholamine concentration during tilt and density of beta-adrenoceptors and tested the effects of beta-adrenergic blockers in the prevention of the vasodepressor reaction in patients with vasodepressor syncope. Ten patients had reproducibly induced vasodepressor syncope (mean 3.1 +/- 0.3 episodes in each patient) with head-up tilt (80 degrees) for 10 minutes with isoproterenol infusion (1 to 3 micrograms/min). Syncope occurred at 6.3 +/- 1.7 minutes during tilt with isoproterenol infusion. The plasma norepinephrine concentration before administration of beta-blocker was significantly elevated during tilt compared to the supine position (0.347 +/- 0.079 ng/ml in supine position with isoproterenol vs 0.468 +/- 0.082 ng/ml at 3 minutes of tilt, p < 0.001, and vs 0.503 +/- 0.106 ng/ml at the onset of vasodepressor reaction, p < 0.005; n = 8). Plasma norepinephrine after administration of the selective beta 1-adrenergic blocker metoprolol (40 mg/day) was similarly elevated (0.282 +/- 0.071 ng/ml in supine position with isoproterenol vs 0.390 +/- 0.078 ng/ml at 3 minutes of tilt, p < 0.05, n = 6; and vs 0.547 +/- 0.152 ng/ml at the onset of vasodepressor reaction, p < 0.001, n = 3).(ABSTRACT TRUNCATED AT 250 WORDS)

[3H]mepyramine binding sites, histamine H1-receptors, in bovine retinal blood vessels

The presence of histamine H1-receptors in the bovine retinal blood vessels was studied with a [3H]mepyramine binding assay. The membranes of purified vessels obtained from bovine retinas showed specific [3H]mepyramine binding sites with a dissociation constant (KD) of 2.78 +/- 0.32 nM. This was similar to values obtained from the retinal neuronal fractions. The binding capacity (Bmax) was 53.8 +/- 1.7 fmol/mg protein, which was about a half that of the retinal neuronal fractions (108.9 +/- 3.1 fmol/mg protein). Some H1-antagonists proved to be potent competitors for [3H]mepyramine binding sites in bovine retinal blood vessels. These results indicate that histamine H1-receptors exist in the retinal blood vessels which may be involved in the physiological and the pathological responses of blood circulation in retinas.