Pharmacology of serotonin receptors causing contraction of isolated bovine posterior ciliary arteries: Role in ocular blood flow

Purpose: To determine the serotonergic (5HT) receptor subtype mediating the contraction of bovine posterior ciliary arteries (BPCAs) in vitro. Methods: Longitudinal isometric tension was measured in BPCA strips (4-5 mm) mounted in 25 mL organ baths containing oxygenated Krebs solution at 37°C. Cumulative contractile concentration-response (C-R) curves were generated for various 5-HT agonists to assess their potencies and maximal degrees of contraction. Multiple agonist C-R curves were also constructed in the presence and absence of receptor-selective antagonists to determine antagonist potencies using Schild plots. Results: Selective and nonselective agonists for 5-HT receptors elicited concentration-dependent contractile responses in BPCAs with the following rank order of potency: MK-212 > BW723C86 > α-methyl-5-HT >5-methoxy-α-5-methyl-5-HT >> R-DO1 > >5-HT >> cabergoline >> 5-methoxy-dimethyl-tryptamine >> 2-methyl-5-HT >> tryptamine. Interestingly, both 8-OH-DPAT (5HT_1A agonist) and quipazine (5HT₃ agonist) did not elicit contractions in BPCAs. The contractions produced by BW723C86 (5-HT_2B agonist) were antagonized by 5-HT receptor blockers, RS-127445 (5-HT_2B antagonist), and M-100907 (5-HT_2A antagonist), yielding antagonist pA₂ values of 7.5 ± 0.12 (n = 4) and 6.2 ± 0.17 (n = 4), respectively. Furthermore, contractions elicited by MK-212 (5-HT_2C agonist) was blocked by RS-102221 (5-HT_2C antagonist), although noncompetitively. Conclusions: On the basis of the pharmacological profile of selective agonists and antagonists, we conclude that serotonin-induced contractions of the BPCA are mediated primarily by a combination of 5HT_2C and/or 5HT_2B receptors. It appears that 5-HT_1A and 5-HT₃ receptors are not involved in the contractile action of BPCAs to serotonin.