Effect of isoprenaline and phenylephrine on the adenosine 3',5'-monophosphate content and mechanical activity of cold-stored and fresh taenia caecum from the guinea-pig

Different types of relationship between beta-adrenergic relaxation and activation of cyclic AMP-dependent protein kinase in canine saphenous and portal veins

We examined the relationships between the relaxation mediated by beta-adrenoceptor and either the associated cyclic AMP production or the activation of cyclic AMP-dependent protein kinase (A-PK) in canine saphenous and portal veins. In the saphenous vein constricted with methoxamine, isoproterenol caused concentration-dependent relaxation (maximum relaxation 92.7%), and concomitant increases in cyclic AMP and A-PK activity ratio (from 52.8 to 73.5%). The portal vein was only slightly relaxed by isoproterenol (14.7% in the longitudinal strips) after constriction with methoxamine, while cyclic AMP and A-PK activation increased significantly. Isoproterenol markedly activated A-PK of the portal vein after KCl constriction (from 52.6 to 74.6%) but the maximum relaxation was only slight (13.3%). The portal vein also showed a smaller relaxation response to either forskolin or dibutyryl cyclic AMP than the saphenous vein. These results indicated that the relationship between the relaxation response to isoproterenol and either cyclic AMP production or A-PK activation was different in the saphenous and portal veins.

Effects of forskolin and isoproterenol on cyclic AMP and tension in the myometrium

The effects of forskolin and isoproterenol on total cyclic AMP accumulation and relaxation were compared in uterine segments from estrogen-treated rabbits and term-pregnant rats. In the rabbit, forskolin (0.2-5.0 microM) inhibited spontaneous and acetylcholine-induced contractions and increased cyclic AMP levels. At lower doses forskolin significantly inhibited contractions but had little effect on cyclic AMP (cAMP) levels. Isoproterenol (0.005-0.5 microM), doses equi-effective to those of forskolin for inhibition of contractions, did not alter cyclic AMP levels. In the rat, forskolin (0.5 microM) and isoproterenol (0.05 microM) inhibited spontaneous and potassium-induced contractions by 80%. This reduction was accompanied by an increase in cyclic AMP levels, but the increase was significantly greater with forskolin than with isoproterenol. We conclude that: (1) cAMP may mediate relaxation under certain conditions but it is not an obligatory mediator, (2) some form of compartmentalization of cyclic AMP might exist in the myometrium.

Effect of prolonged cold storage on the mechanical activity of rabbit atria

Even after 21 days of cold storage at 2 degrees C, rabbit atria still showed a detectable response only to isoproterenol while other cardiostimulants (norepinephrine, ouabain, anthopleurin-A and nicotine) failed to activate tissues stored up to 14 days. The specific fluorescence of catecholamines disappeared from the tissue after 5 days of cold storage. The maximum decrease in oxygen consumption and tissue catecholamine content was observed after 14 days and a concomitant decrease in tissue ATP content, Na+-K+, Ca2+ and Mg2+-ATPases was nearly maximal after 7 days of cold storage. The cold storage had no apparent effect on the ultrastructures of myocardial cells except a disappearance in glycogen granules.

Stimulation of beta-adrenoceptors in the exercising human forearm

In order to study the effects of local stimulation of beta-adrenoceptors on limb blood flow during exercise and on muscle metabolism, the non-selective beta-adrenoceptor agonist, isoprenaline, was infused intra-arterially into the forearm of six healthy, young subjects. Isoprenaline caused a marked increase in forearm blood flow not only at rest but also during dynamic forearm exercise. The forearm release of lactate increased considerably during drug infusion whereas glucose uptake decreased. The net uptake of free fatty acids during exercise decreased. There were no systemic effects of isoprenaline. These findings suggest that the resistance vessels are responsive to beta-adrenergic vasodilatation also during muscle exercise. The increase in blood flow through the exercising forearm may be secondary to an increased muscle metabolism. The increased lactate release from the exercising muscles indicates an increased muscle glycogen breakdown during beta-adrenoceptor stimulation.

Electrophysiological and cyclic AMP responses to beta-adrenergic receptor stimulation and catecholamine levels in cat and rabbit sympathetic ganglion

The effects of beta 1- and beta 2-selective and nonselective adrenergic agonists and antagonists were studied in the cat and rabbit superior cervical ganglion (SCG) in situ and in vitro. In experiments on the cat SCG in situ beta-adrenergic agonists induced an increase in positive afterpotential (PAP) of compound ganglionic action potential evoked by preganglionic stimulation in the following order of potency: isoprenaline (ISO) greater than salbutamol much greater than tazolol. Beta-adrenergic antagonists inhibited the ISO response in the following order of potency: propranolol greater than exaprolol much greater than H 35/25. Practolol did not exhibit beta-adrenergic blocking action; on the contrary, in higher doses it enhanced the response to ISO. In higher doses all beta-adrenergic antagonists tested depressed ganglionic transmission in an approximately similar range of doses. In experiments in vitro, using sucrose-gap technique. ISO induced depolarization of the cat SCG and this effect was inhibited in a dose-dependent manner by propranolol. Beta-adrenergic agonists did not induce depolarization of the rabbit SCG in experiments in situ and in vitro. The level of noradrenaline and dopamine in the cat and rabbit SCG did not differ significantly. Low level of adrenaline was found in the cat SCG; however, no adrenaline was detected in the rabbit SCG. No statistically significant change of cyclic AMP content was observed in the cat and rabbit SCG or in the incubation medium under conditions similar to those in which ISO induced depolarization of the cat SCG. It is concluded that the beta-adrenergic receptors present in the cat SCG resemble the beta 2-subtype and that the presence of an electrophysiologic response to beta-adrenergic agonists appears to be species-dependent. The depolarization of the cat SCG due to beta-adrenergic receptor stimulation is not coupled with cyclic AMP increase.

The mechanism of the relaxant effect of 2-2'-pyridylisatogen on the isolated taenia of the guinea-pig caecum

1 2-2'-Pyridylisatogen tosylate (PIT) slowly relaxed taenia caeci preparationg of the guinea-pig in a concentration-dependent manner (threshold 2.5 muM). The relaxant effect did not show tachyphylaxis.2 The relaxation was not affected by tetrodotoxin (0.3 muM), guanethidine (17 muM) nor by a combination of phentolamine (36 muM) and propranolol (4 muM)3 In taenia caeci preparations suspended in K(+)-depolarizing, Ca(2+)-free Ringer, addition of Ca(2+) (0.1 to 30 mM) resulted in a slow contraction. PIT (50 muM) and papaverine (15 muM) antagonized these contractions, whereas indomethacin (28 muM) was ineffective.4 Although PIT (50 muM for 30 min) caused a relaxation of the taenia, and, when the tone of the preparations was restored with carbachol, antagonized adenosine 5'-triphosphate (ATP)-induced relaxations, relaxation of the taenia with papaverine (30 muM for 5 min) did not antagonize ATP-induced relaxations. It is concluded that the relaxant and ATP-receptor blocking actions of PIT are independent properties of the compound.