Interaction between phenylephrine and prostaglandins E1 and F1 alpha on the contractile responses of vascular muscle

Effects of prostaglandin E1, isoproterenol and forskolin on cyclic AMP levels and tension in rabbit aortic rings

The role of cyclic AMP in the control of vascular smooth muscle tone was studied by monitoring the effects of prostaglandin E1 (PGE1), isoproterenol and forskolin on cyclic AMP levels and tension in rabbit aortic rings. PGE1, isoproterenol and forskolin all increased cyclic AMP levels in rabbit aortic rings. Isoproterenol and forskolin relaxed phenylephrine-contracted aortic rings, but PGE1 contracted the rings in the presence or absence of phenylephrine. Isoproterenol relaxed these PGE1-contracted aortic rings without further change in total cyclic AMP levels, which were already elevated by the PGE1 alone. Pretreatment with forskolin potentiated the effects of PGE1 on cyclic AMP levels. PGE1 caused contractions in muscles partially relaxed by forskolin, even though very large increases in cyclic AMP levels (30 fold) were produced by PGE1 in the presence of forskolin. Isoproterenol was able to relax these forskolin-treated, PGE1-contracted muscles with no further increase in cyclic AMP levels. Thus, there does not appear to be a good correlation between total tissue levels of cyclic AMP and tension in these experiments. Our results suggest that, if cyclic AMP is responsible for relaxation of smooth muscle, some form of functional compartmentalization of cyclic AMP must exist in this tissue.

The effect of prostaglandins F2 alpha and E2 on the adrenergic activity of the uterus of laying hens (Gallus domesticus) in vitro

The effect of PGF2 alpha and PGE2 before or after administration of isoproterenol or norepinephrine was studied on uterine strips of White Leghorn laying hens. Prostaglandins (0.2 ng/ml of bath solution) always produced an increase in tension and area under the tracing, the effect of PGF2 alpha being significantly marked especially when administered after norepinephrine. Norepinephrine and isoproterenol (0.1 microgram/ml of bath solution) produced a marked decrease in area under the tracing, the effect of isoproterenol being more potent especially when administered before prostaglandins.

Effect of in vitro administration of captopril on vascular reactivity of rat aorta

The effect of acute administration of captopril, an angiotensin converting enzyme inhibitor, on vascular responses of rings of rat aortic smooth muscle was tested in vitro. Dose-response curves for various vasoactive agents were obtained before and after exposure to captopril (2 x 10(-4) M) for 30 minutes. In the presence of captopril, contractile responses to angiotensin I (5 x 10(-10) to 5 x 10(-8) M) were attenuated significantly, probably as a result of decreased local conversion of angiotensin I to angiotensin II. Contractile responses to angiotensin II (10(-11) to 5 x 10(-9) M) were not affected by captopril. All responses to norepinephrine (10(-9) to 10(-4) M) and phenylephrine (10(-8) to 10(-4) M) were attenuated significantly from control in the presence of captopril. In the presence of the alpha-adrenergic antagonist, phentolamine, captopril did not affect either the contractile responses to KCl (30 to 100 mM) or the isoproterenol-induced (10(-8) to 10(-5) M) relaxation of KCl-depolarized tissue. These results suggest that captopril decreased vascular responsiveness to alpha-adrenergic agonists but not to beta-adrenergic agonists. Low concentrations of bradykinin (10(-10) to 10(-8) M) induced contraction in KCl-depolarized tissue while higher concentrations (10(-7) and 10(-6) M) induced relaxation. In the presence of captopril, relaxation occurred at all concentrations of bradykinin (10(-10) to 10(-6) M), probably as a result of decreased degradation of the bradykinin. These data suggest depression of alpha-adrenergic responsiveness in vascular smooth muscle as another potential antihypertensive action of captopril.

Inhibitory effects of some catecholamines on contractions of uterine strips isolated from estrous and spayed rats. Influence of endogenous and exogenous prostaglandins on the action of methoxamine

Cumulative dose-response curves were produced for the effect of different adrenergic agonists on the contractions of uterine strips from natural estrous and ovariectomized rats. Isoproterenol, norepinephrine and phenylephrine inhibited uterine spontaneous contractions, whilst methoxamine stimulated them. The inhibitory influences were blocked by propranolol and the excitatory effect of methoxamine was abolished by phentolamine. However, the sensitivity of the rat uterus to beta- and alpha-adrenergic agonists varied depending on the hormonal state i.e. strips from rats in natural estrus were more sensitive to inhibitory effect of beta-agonist than were those from ovariectomized animals. On the contrary, the apparent potency and the efficacy of an alpha-agonist (methoxamine) was higher in ovariectomized uterine strips than in those from estrous rats. Indomethacin, at a concentration known to inhibit the endogenous synthesis of prostaglandins, depressed the spontaneous activity of strips from spayed rats and prevented the response to methoxamine, whereas in strips from natural estrous rats, the drug was unable to modify either the spontaneous contractions or the stimulating action of methoxamine. In addition in the presence of threshold doses of PGE1, PGE2 and F2 alpha the dose-response curve to methoxamine shifted to the left for uterus from ovariectomized animals. The results suggest that uterine strips from estrous rats have more affinity to the effect of beta-adrenergic agonists, whereas those from spayed animals were more sensitive to the effect of an alpha-adrenergic agent. Also the effectiveness of an alpha-adrenergic agonist, methoxamine, appears to be related to unaltered tissue prostaglandins.

Effects of prostaglandin E1 on contractility and 45Ca release in rabbit aortic smooth muscle

Concentrations of prostaglandin E1 (PGE1; 10(-7) M) that do not elicit tension responses in aortic strips potentiate contractions induced by submaximal concentrations (10(-8)-10(-7) M) of norepinephrine (NE) or angiotensin III (Ang III) but not those of high K+ depolarization or maximal NE or Ang III concentrations. Higher concentrations of PGE1 (10(-6) M and above) initiate contractions which are additive with submaximal responses to NE and Ang III but not to K+. These same concentrations of PGE1 also decrease 45Ca retention at high affinity La+++-resistant sites in a manner similar to but not additive with NE and Ang III. Uptake of 45Ca at low affinity La+++-resistant sites (which is increased by high K+-depolarization) is not altered by 10(-6) M PGE1. The effects of PGE1 are not altered by decreased extracellular Ca++ (0.1 mM), decreased temperature, phentolamine or meclofenamate. Thus, PGE1 does not appear to increase uptake of extracellular Ca++ in this smooth muscle tissue. Instead, PGE1 increases mobilization of Ca++ from the same high affinity La+++-resistant sites affected by Ang III and NE and, in this manner, may increase responses to these two stimulatory agents.