Participation of catecholamine in the digitoxigenin-induced contraction of isolated dog veins

Vascular smooth muscle response to ouabain. Relation of tissue Na+ to the contractile response

Smooth muscle responses to Na+ pump inhibition are thought to reflect two elements: a neurogenic contribution, involving catecholamine release from nerve terminals, and a myogenic response, attributed to relations between pump activity, [Na+]i, and [Ca2+]i. In the present study, we describe the time course and magnitude of cell Na+ changes, assessed by two methods, atomic absorption and nuclear magnetic resonance spectroscopy during the myogenic contractile response of rabbit aorta strips to ouabain. A threshold concentration of 3 x 10(-7) mol/L induced a gradual rise in [Na+]i. Both methods showed an essentially identical monotonic rise over 4 to 8 hours from a baseline level of 8 to 10 mmol/L water to a peak, which was approximately fivefold higher. The neurogenic (rapid) and myogenic (delayed and gradual) contractile responses were temporally distinct. Ouabain at 10(-7) mol/L, a concentration 10- to 100-fold lower than the threshold for catecholamine-dependent rapid-onset responses, induced only a delayed and gradual contractile response, which reached a maximum at 6 to 8 hours. With 10(-6) mol/L ouabain, the delayed response of 1.6 +/- 0.2 g peaked at 7.3 +/- 1.1 hours and was sustained for 16 hours. The time course was similar to that for change in [Na+] but somewhat later. Ouabain at 10(-5) and 10(-4) mol/L induced a delayed response that was identical in magnitude but also induced an early rapid contractile response, which was prevented by reserpine or phentolamine pretreatment. These agents did not influence the delayed response.(ABSTRACT TRUNCATED AT 250 WORDS)

Vasoconstriction induced by ouabain in the canine coronary artery: contribution of adrenergic and nonadrenergic responses

Ouabain, when applied to rings of the left circumflex coronary artery of the dog (which contains both alpha 1-adrenoceptors leading to contraction and beta 1-adrenoceptors leading to relaxation) caused an initial contraction which peaked within 15 minutes and a later secondary increase in tension which peaked within 60 minutes. These contractions were prevented by Ca2+ removal or by verapamil. Adrenergic denervation with 6-hydroxydopamine did not affect the initial contraction. Thus it is due to a nonadrenergic effect of the glycoside. Since the secondary increase in tension was prevented by adrenergic denervation and prazosin, it is likely to be due to norepinephrine released from adrenergic nerves acting on alpha-adrenoceptors. This interpretation was confirmed by the finding that ouabain, after a latent period of about 35 minutes, augmented the output of 3H-norepinephrine from helical strips of the artery previously incubated with tritiated transmitter. In rings contracted with prostaglandin F2 alpha, ouabain reduced beta-adrenergic relaxations caused by isoproterenol or exogenous norepinephrine, but not those caused by sodium nitroprusside. Thus, in this artery, ouabain depresses the responses of the beta-adrenoceptors to the norepinephrine which it releases, thereby permitting the neurotransmitter to cause contraction by activating postjunctional alpha 1-adrenoceptors.

Analysis of ouabain-induced contractions in isolated coronary arteries

Contractile responses to ouabain in helical strips of dog and monkey coronary arteries were investigated. Ouabain (5 X 10(-8) to 5 X 10(-6) M) caused a dose-related contraction in dog and monkey arteries; the response of monkey coronary arteries was significantly greater. In dog coronary arteries, contractile responses to high concentrations of ouabain were potentiated by treatment with propranolol. In the arteries contracted with ouabain, the addition of phentolamine caused a relaxation. Contractile responses of dog coronary arteries to ouabain were markedly suppressed by exposure to Ca2+-free media or by treatment with verapamil. Reduction of external concentration of K+ or lowering the temperature of bathing media did not selectively influence the ouabain-induced contraction. These results suggest that ouabain-induced contractions of dog coronary arteries are associated mainly with an increase in the Ca2+-influx, which does not result from an inhibition of the Na+, K+-activated ATPase nor from an activation of alpha adrenoceptors by noradrenaline released from adrenergic nerves. Ouabain in high concentrations seems to liberate noradrenaline from adrenergic nerves, which preferentially activates beta adrenoceptors in dog coronary artery.

Potentiating and depressive effects of ouabain and potassium-free solutions on rat mesenteric resistance vessels

We have investigated the in vitro effects of ouabain and K-free solutions on some pharmacological and electrophysiological properties of rat mesenteric resistance vessels (internal diameter approximately 190 micrometers). Vessels were mounted as ring preparations on a myograph capable of measuring their isometric wall tension. In normal saline solutions, vessels did not exhibit any tone and had a membrane potential of -54 mV. Both 1 mM ouabain and K-free solutions caused a transient depolarization of 5-8 mV; thereafter the membrane slowly depolarized to about -45 mV after 30 minutes. There was no mechanical response to ouabain, but K-free solutions caused a transient development of tension which could be inhibited by phentolamine (1 microM). In norepinephrine-activated vessels, exposure to ouabain or K-free solutions caused a small depolarization and an increase in tension. Long-term (30-minute) exposure to 1 mM ouabain or K-free solutions reduced the amplitude of norepinephrine responses and, for the lower (but not the higher) norepinephrine concentrations, the membranes were about 14 mV more depolarized than control. The mechanical responses to a cocktail of norepinephrine in a high potassium solution were, however, unaffected. Re-exposure to normal saline solution produced a transient hyperpolarization and transiently eliminated the norepinephrine response, but thereafter the membrane potential and response returned to normal. The results indicate that ouabain and K-free solutions can have both short-term potentiating and long-term depressive effects on the mechanical response of rat mesenteric resistance vessels to norepinephrine.

The effects of acetylstrophanthidin and ouabain on the sympathetic adrenergic neuroeffector junction in canine vascular smooth muscle

We performed experiments to determine the effects of acetylstrophanthidin (ACS) and ouabain on the adrenergic neuroeffector junction in dog saphenous veins. In quiescent strips incubated with 3H-norepinephrine (3H-NE), the drugs caused contraction and a progressive increase in overflow of 3H-NE and O-methylated metabolites; 3,4-dihydroxyphenylglycol (DOPEG) decreased. Tissue uptake of 3H-NE was partially inhibited. After surgical sympathectomy, both contraction and 3H-NE overflow were markedly attenuated. Following chemical sympathectomy with 6-hydroxydopamine, ouabain contractions were 11% of control, whereas the contractions due to exogenous norepinephrine were exaggerated. The initial overflow of 3H-NE was unaffected by tetrodotoxin, but the later and larger overflow with prolonged exposure was depressed. The former occurred in the absence of Ca2+, but the latter was Ca2+ dependent. Inhibition of the neuronal amine carrier by cocaine or desipramine and blockade of the neuronal alpha-adrenoceptors with phentolamine or phenoxybenzamine attenuated the release of 3H-NE evoked by ACS and ouabain. During electrical stimulation, ACS augmented the overflow of 3H-NE. This was attenuated by cocaine, desipramine, and the alpha-adrenolytic drugs. ACS, like pargyline, augmented the overflow of 3H-NE evoked by tyramine and depressed that of DOPEG. These experiments suggest that acetylstrophanthidin and ouabain (1) cause contraction of vascular smooth muscle by displacement of norepinephrine from neuronal stores, (2) reduce neuronal monoamine oxidase activity, (3) facilitate and may trigger Ca2+-dependent exocytotic release of norepinephrine, (4) partially inhibit the neuronal amine carrier mechanism but do not interfere with extraneuronal disposition of norepinephrine, and, finally (5) may have unexplained interactions with prejunctional alpha-adrenoceptors.

Effects of ouabain and potassium-free solution on the contraction of isolated blood vessels

Ouabain and K-free solution affected the tension development of isolated vessels as follows. In rabbit aorta, ouabain induced a transient contraction followed by a delayed sustained contraction whereas K-free solution was almost ineffective; both ouabain and K-free solution produced a sustained contraction in guinea pig aorta; rat aorta rarely responded to ouabain but developed a contraction in K-free solution. Phentolamine inhibited the transient contraction produced by ouabain in rabbit aorta. In rabbit aorta, ouabain produced a concentration-dependent potentiation of norepinephrine contraction. In denervated rabbit aorta, however, the potentiation only occurred in the presence of 5 X 10(-7) to 5 X 10(-6) M ouabain. K-free solution inhibited the norepinephrine contraction in denervated rabbit aorta. Ouabain potentiated the norepinephrine contraction in guinea pig aorta but not in rat aorta. It is concluded that ouabain and K-free solution affect contraction both indirectly via the release of endogenous catecholamines and directly by acting on vascular smooth muscle, and that there are species differences in the sensitivity to ouabain.