The isolated rat portal vein as a model for studying beta-adrenoreceptor agonists and antagonists

1. The isolated rat portal vein was employed for essaying beta-agonists and beta-antagonists. beta-agonists elicit a log-dose dependent reduction of force of the spontaneous, rhythmic, myogenic contractions of the vein. The ID50 dose for isoprenaline was 8.40 +/- 0.04 SEM (negative log. of the molar concentration). Isoprenaline was the most potent of the beta-agonists. If given the value 100, then fenoterol is 75; salbutamol 10; terbutaline 6.75 and during alpha-blockade, adrenaline 28.80 and noradrenaline 0.27. The ratio of potency is thus ISOP greater than FENOT greater than ADR greater than SALB greater than TERB greater than NOR. This suggests that the predominant beta-receptors are beta 2. 2. The duration of effects was shortest for isoprenaline and largest for fenoterol and terbutaline. 3. With the highly potent, specific, competitive beta 2-blocker ICI 118551, a pA2 against isoprenaline of 9.30 (+/- SEM 0.03; C.L. 95% 9.22-9.38) was found. The pA2 against fenoterol, terbutaline, adrenaline and noradrenaline were included in these confidence-limits. With propranolol, a pA2 against isoprenaline of 8.82 (+/- SEM 0.02) was obtained. 4. The high pA2 found against non-selective (isoprenaline, adrenaline) and selective (fenoterol, terbutaline) beta 2-agonists is, as said, practically the same. This, together with the potency ratio ISOP greater than ADR greater than NOR, markedly suggest that the population of beta-receptors in the smooth muscle of the rat portal vein is homogeneously beta 2. 5. The method is very useful to essay beta-agonists and beta-antagonists. As straight regression lines with very high correlation coefficients are obtained, the potency and efficacy of agonists may be relatively easily obtained.