Stimulation of vascular smooth muscle sodium, potassium--adenosinetriphosphatase by vasodilators

Digoxin inhibits development of hypoxic pulmonary hypertension in mice

Chronic hypoxia is an inciting factor for the development of pulmonary arterial hypertension. The mechanisms involved in the development of hypoxic pulmonary hypertension (HPH) include hypoxia-inducible factor 1 (HIF-1)-dependent transactivation of genes controlling pulmonary arterial smooth muscle cell (PASMC) intracellular calcium concentration ([Ca(2+)](i)) and pH. Recently, digoxin was shown to inhibit HIF-1 transcriptional activity. In this study, we tested the hypothesis that digoxin could prevent and reverse the development of HPH. Mice were injected daily with saline or digoxin and exposed to room air or ambient hypoxia for 3 wk. Treatment with digoxin attenuated the development of right ventricle (RV) hypertrophy and prevented the pulmonary vascular remodeling and increases in PASMC [Ca(2+)](i), pH, and RV pressure that occur in mice exposed to chronic hypoxia. When started after pulmonary hypertension was established, digoxin attenuated the hypoxia-induced increases in RV pressure and PASMC pH and [Ca(2+)](i). These preclinical data support a role for HIF-1 inhibitors in the treatment of HPH.

Serine 68 phospholemman phosphorylation during forskolin-induced swine carotid artery relaxation

BACKGROUND

Phospholemman (PLM) is an abundant phosphoprotein in the plasma membrane of cardiac, skeletal and smooth muscle. It is a member of the FXYD family of proteins that bind to and regulate the Na,K-ATPase. Protein kinase A (PKA) is known to phosphorylate PLM on serine 68 (S68), although the functional effect of S68 PLM phosphorylation is unclear. We therefore evaluated S68 PLM phosphorylation in swine carotid arteries.

METHODS

Two anti-PLM antibodies, one to S68 phosphorylated PLM and one to unphosphorylated PLM, were made to PLM peptides in rabbits and tested with purified PLM and PKA-treated PLM. Swine carotid arteries were mounted isometrically, contracted, relaxed with forskolin and then homogenized. Proteins were separated on SDS gels and the intensity of immunoreactivity to the two PLM antibodies determined on immunoblots.

RESULTS

The antipeptide antibody 'C2' primarily reacted with unphosphorylated PLM, and the antipeptide antibody 'CP68' detected S68 PLM phosphorylation. Histamine stimulation of intact swine carotid artery induced a contraction, increased the CP68 PLM antibody signal and reduced the C2 PLM antibody signal. High extracellular [K(+)] depolarization induced a contraction without altering the C2 or CP68 PLM signal. Forskolin-induced relaxation of histamine or extracellular [K(+)] contracted arteries correlated with an increased CP68 signal. Nitroglycerin-induced relaxation was not associated with changes in the C2 or CP68 PLM signal.

CONCLUSIONS

These data suggest that a contractile agonist increased S68 PLM phosphorylation. Agents that increase [cAMP], but not agents that increase [cGMP], increased S68 PLM phosphorylation. S68 PLM phosphorylation may be involved in cAMP-dependent regulation of smooth muscle force.

Semicarbazide-sensitive amine oxidase: role in the vasculature and vasodilation after in situ inhibition

1. The characteristics of semicarbazide-sensitive amine oxidase (SSAO) are reviewed and the unknown physiological or pathological role of this enzyme emphasized. 2. The various mechanisms of action proposed for the vasodilator drug hydralazine are considered. In particular, the inhibitory action on various enzymes, related or not to cardiovascular function, are discussed. 3. Studies linking inhibition of SSAO to hydralazine hypotension are reviewed and a general hypothesis relating both actions is presented. The hypothesis postulates that (a). vascular SSAO is involved in the regulation of vascular tone, and (b). hydralazine vasodilation is the consequence of vascular SSAO inhibition. 4. Evidence supporting these postulates is presented and vascular SSAO inhibition is proposed as a novel mechanism of vasodilation.

Role of K+ channel opening and Na(+)-K+ ATPase activity in airway relaxation induced by salbutamol

To determine the role of K+ channel opening and Na(+)-K+ ATPase activity in the beta-adrenoceptor-mediated relaxation of airway smooth muscle, we studied canine bronchial segments under isometric conditions in vitro. Relaxant responses to salbutamol were not altered by glibenclamide or apamin but inhibited by charybdotoxin, where significant inhibition was observed only at salbutamol concentrations of less than 10(-6) M. In contrast, only the relaxations induced by salbutamol at 3 x 10(-6) M and greater were sensitive to ouabain. Relaxations produced by low and high concentrations of salbutamol were selectively attenuated by charybdotoxin and ouabain, respectively, in a concentration-dependent manner. These results suggest that both Ca(2+)-activated K+ channel and Na(+)-K+ ATPase may be operative in the airway relaxation induced by low and high concentrations of the beta-adrenergic agonist, respectively.

Beta-adrenergic effects on cellular Na, Mg, Ca, K and Cl in vascular smooth muscle: electron probe analysis of rabbit pulmonary artery

The effects of beta-adrenergic stimulation on the cellular content and subcellular distribution of Na, Mg, Ca, K and Cl were determined by electron probe X-ray microanalysis of muscles stimulated with 5-hydroxytryptamine. Isoproterenol caused a significant decrease in cytoplasmic and mitochondrial Na and Cl, and an increase in cytoplasmic Mg. Isoproterenol also significantly decreased total cytoplasmic Ca measured with small diameter probes, without affecting cellular Ca measured with large probes that included the sarcoplasmic reticulum (SR). The decrease in cytoplasmic Na and the effects on cytoplasmic and cellular Ca are consistent with, respectively, beta-adrenergic stimulation of the Na-pump and of Ca-uptake into the SR, but the beta-adrenergic increase in cytoplasmic Mg also raises the possibility of stimulated Na/Mg exchange.

Comparison of the effects of hydralazine and nifedipine on contractions and 45Ca influx of rat aorta

The effect of the vasodilator hydralazine has been compared with nifedipine on KCl-(K+) (60 mM) and noradrenaline-(NA) (10 microM) induced 45Ca uptake and contractile responses in rat aorta arterial strips without endothelium. Hydralazine (0.5-10 mM) was equally effective in relaxing K(+)- (IC50 = 2.2 +/- 0.17 mM) and NA- (IC50 = 3.06 +/- 0.25 mM) induced tension, the degree of relaxation depending on the dose. Nifedipine totally inhibited K(+)- (IC50 = 3.16 +/- 0.28 nM) induced contractions with lower doses than were necessary to relax (up to 54.0 +/- 4.1% with supramaximal concentrations) NA-induced contractions (IC50 = 1.48 +/- 0.12 microM). In the experiments in a calcium-free medium, nifedipine (1 microM) had no effect on the NA- (10 microM) induced contractions whereas hydralazine (1 mM) strongly inhibited them. Nifedipine did not affect the basal uptake of 45Ca but the induced uptakes were reduced to 66.3 +/- 3.2% (K+) and 65.5 +/- 4.1% (NA) of their basal values. Hydralazine did not affect the basal uptake of 45Ca nor that induced by the two vascoconstrictor agents. These results suggest that nifedipine acts on the cell membrane by blocking the movements of calcium through the voltage-dependent and receptor-operated calcium channels, whilst hydralazine has an intracellular effect.

Future directions in vasodilator therapy for heart failure

Vasodilator therapy has become a major pharmacologic approach for improving left ventricular function, and consequently, vasodilator drugs are being used increasingly in the treatment of heart failure. Ideally, vasodilator drugs used in the long-term management of heart failure should show clearly defined pharmacodynamic effects. These include reduced impedance to left ventricular ejection, increased venous capacitance, increased left ventricular ejection fraction and reduced heart size, absence of neurohormonal stimulation, and slowed progression of left ventricular dysfunction. The mechanisms of action and sites of activity of the various vasodilator drugs currently available vary considerably, and none as yet has proved ideal for the treatment of heart failure or hypertension. The complexity surrounding the multiple vasoconstrictor mechanisms involved in heart failure has led to a rationale for combined vasodilator therapy and certain combinations are discussed. From a therapeutic standpoint, the development of drugs with multiple mechanisms of action is particularly attractive. Flosequinan is a new vasodilator agent whose cellular mechanism of action remains uncertain. Flosequinan has the advantage of being able to relax both arterial and venous beds and as such may be particularly beneficial in the treatment of heart failure.

The vasorelaxant effects of milrinone and other vasodilators are attenuated by ouabain

The purpose of this study was to investigate the effect of ouabain pretreatment (1 microM) on relaxation induced by milrinone and other reference vasorelaxants in guinea pig aorta. Pretreatment with ouabain for 1 h significantly reduced the threshold concentration and increased the vasoconstrictor potency of phenylephrine. Relaxation by milrinone of aortic rings contracted by phenylephrine or by an equieffective concentration of phenylephrine in the presence of ouabain was significantly attenuated in the presence of ouabain. The effect of all other vasorelaxants tested, which included isosorbide dinitrate, hydralazine, sodium nitroprusside, forskolin, HA 1004, isoproterenol and verapamil, were also significantly reduced in the presence of ouabain. The vasorelaxant effects of milrinone and verapamil were also evaluated in K+-contracted guinea pig aorta. In contrast to results obtained with phenylephrine-contracted vessels, milrinone and verapamil were equipotent as vasorelaxants in K+-contracted vessels in the presence or absence of ouabain. The results show that ouabain not only potentiates the effect of the vasoconstrictor phenylephrine, but also reduces the potency of drugs that cause vasorelaxation in phenylephrine-contracted tissues, regardless of the mechanism of action of the vasorelaxant. These data may have clinical relevance to the concomitant use of vasodilators and digitalis in the treatment of congestive heart failure.

Membrane properties of bovine airway smooth muscle cells: effects of maturation

Airway smooth muscle cells of bovine trachealis muscle from immature (2 weeks old), developing (5 months old) and mature animals (greater than 5 years old) have resting membrane potentials of -63.5 +/- 0.4 mV, -62.2 +/- 0.8 mV and -60.3 +/- 0.7 mV, respectively. These resting potentials were not significantly different. The contribution of the electrogenic sodium pump to the resting membrane potential of airway smooth muscle cells decreases significantly with age being 38.3% +/- 0.9, 30.6% +/- 0.8 and 21.3% +/- 0.9 in tissues from immature, developing and mature cows, respectively. The contribution of the electrogenic sodium pump to resting membrane potential was not influenced by the degree of stretch (applied load) of the airway smooth muscle cells. However, resting membrane potential was reduced with increasing tone especially when loads between 5 to 10 g were applied. The densities of Na+/K+ pump sites, as measured by [3H] ouabain binding, decreased significantly as a consequence of maturation and were 4.12 +/- 5.23, 3.08 +/- 0.26 and 1.94 +/- 0.38 pmoles/mg protein ouabain binding in tissues from immature, developing and mature animals respectively. The affinity of the pump sites for the cardiac glycoside, ouabain, did not change during maturation. We conclude that maturation alters both the number of Na+/K+ pump sites and the contribution of the electrogenic sodium pump to the membrane potential of airway smooth muscle cells. These age related changes may contribute to the reduced airway reactivity to both bronchoconstrictor and bronchodilator agents previously observed both in vitro and in vivo.

Beta-adrenergic relaxation of smooth muscle: differences between cells and tissues

The present studies were carried out in an attempt to resolve the controversy about the Na+ dependence of beta-adrenergic relaxation in smooth muscle. Previous studies on isolated smooth muscle cells from the toad stomach had suggested that at least some of the actions of beta-adrenergic agents, including a stimulatory effect on 45Ca efflux, were dependent on the presence of a normal transmembrane Na+ gradient. Studies by other investigators using tissues derived from mammalian sources had suggested that the relaxing effect of beta-adrenergic agents was Na+ independent. Uncertainty remained as to whether these discrepancies reflected differences between cells and tissues or differences between species. Thus, in the present studies, we utilized both tissues and cells from the same source, the stomach muscle of the toad Bufo marinus, and assessed the Na+ dependence of beta-adrenergic relaxation. We found that elimination of a normal Na+ gradient abolished beta-adrenergic relaxation of isolated cells. In tissues, however, similar manipulations had no effect on relaxation. The reasons for this discrepancy are unclear but do not appear to be attributable to changes in smooth muscle function following enzymatic dispersion. Rather they may be attributable to unique properties of tissues, such as the presence of pacemaker cells, nerves, and so on. Thus the controversy concerning the mechanism of beta-adrenergic relaxation may reflect inherent differences between tissues and cells.

Mechanism of action of natriuretic fraction of urine

The nature of calcium dependence of natriuretic fraction (NF)-induced contractions in the rat smooth muscle anococcygeus and a possible correlation between the effect of NF on sodium transport and its natriuretic potency was investigated. NF and noradrenaline-induced contractions were partially dependent on external calcium concentration. Ouabain and potassium (K) free solution (KoPSS)-induced contractions were totally dependent on external calcium and were more effectively inhibited by calcium antagonists than those of NF and noradrenaline suggesting pharmacomechanical coupling as the mode of calcium dependence of NF. Like other agonists which contract by pharmacomechanical coupling, NF stimulated sodium transport (86rubidium uptake) in dog saphenous vein. Such stimulation correlated positively (r = 0.495, n = 17, P less than 0.05) with the natriuretic potency of NF. NF, like noradrenaline, contracts smooth muscle by pharmacomechanical coupling and NF-induced natriuresis is associated with stimulation of the sodium pump.

Effect of captopril on Na-K ATPase and Mg ATPase activities of erythrocyte ghost membranes

Captopril has a known hypotensive action derived from the inhibition of the converting enzyme (Kininase II). To investigate whether other mechanism may contribute to vasodilation, Na-K ATPase and Mg ATPase activities on ghost red blood cells membranes were examined in the presence of the drug. The results show a stimulation of Mg ATPase activity and an inhibition of Na-K ATPase activity in a concentration dependent manner. The latter action does not seem to be related to a vasodilator effect.

Calcium-induced pulmonary vasodilation: modification by meclofenamate and ouabain

Based on work with isolated arteries, we hypothesized that elevation of the extracellular calcium concentration would cause pulmonary vasodilation by stimulation of the Na+K+ATPase. If so, ouabain should inhibit the "membrane stabilizing" effect of a high calcium concentration. Addition of CaCl2 to the perfusate of rat isolated lungs to give a final concentration of 10(-2)M caused vasodilation when the lungs were vasoconstricted by KCl, angiotensin II or hypoxia. Ouabain addition to the perfusate increased pulmonary artery perfusion pressure and the magnitude of angiotensin II-induced vasoconstriction delayed vasodilation and transformed the CaCl2-induced vasodilation into rapid vasoconstriction accompanied by edema formation. Meclofenamate reduced the calcium-induced vasodilation suggesting that part of the high calcium vasodilation may be due to the action of a vasodilator prostaglandin. The vasodilation due to prostacyclin was blocked in lungs treated with ouabain. The results taken together suggest that high calcium vasodilation operates by mechanisms which may be prostaglandin-dependent and prostaglandin-independent, both of which can be blocked by ouabain.

Beta-adrenergic effects on transmembrane 45Ca fluxes in isolated smooth muscle cells

The effect of the beta-adrenergic agent isoproterenol (ISO) on transmembrane fluxes of 45Ca was assessed in smooth muscle cells in suspension. Exposure of the cells to this smooth muscle relaxant provoked a large (13-fold) transient increase in 45Ca efflux from the isolated cells. This effect was blocked by the beta-adrenergic antagonist pindolol and mimicked by exposure of the cells to dibutyryl adenosine 3',5'-cyclic monophosphate. ISO had no significant effect on 45Ca binding or on 45Ca influx in those cells, thus the ISO-induced increase in 45Ca efflux appears to reflect increased Ca2+ extrusion from the smooth muscle cells. Because the ISO-induced increase in 45Ca efflux was abolished by pretreatment of the cells with the Na+-K+ pump inhibitor ouabain and by lowering extracellular Na+ levels, we propose that the increased Ca2+ extrusion in response to ISO is coupled to increased activity of the plasmalemmal Na+-K+ pump via increased Na+-Ca2+ exchange.

Beta-adrenergic stimulation of 42K influx in isolated smooth muscle cells

The present studies were designed to examine the effect of the beta-adrenergic agent isoproterenol (ISO) on 42K influx into isolated smooth muscle cells. Unidirectional 42K influx studies were carried out as described previously using suspensions of smooth muscle cells derived from the toad stomach. Results from these studies indicated that ISO (10(-4) M) produces an approximate threefold increase in 42K influx within 1 min after drug addition. This response is transient such that the flux rate returns to near basal levels within 5 min after drug addition. The stimulatory effect of ISO on 42K influx is blocked by pretreatment with ouabain (1-7.5 X 10(-4) M). ISO thus appears to increase 42K influx by stimulating the Na+-K+ pump in the smooth muscle cells. Other findings indicate that the stimulatory effect of ISO on Na+-K+ pumping is dose dependent, blocked by the beta-antagonist pindolol, and mimicked by dibutyryl adenosine 3',5'-cyclic monophosphate (cAMP). These findings support our hypothesis that ISO elicits a beta-adrenergic, cAMP-dependent activation of the Na+-K+ pump in isolated smooth muscle cells.

Stimulation of Na+,K+-ATPase of isolated smooth muscle membranes by the Ca2+ channel inhibitors, nimodipine and nitrendipine

Nimodipine (0.015 to 1.5 microM) increased Na+, K+-ATPase activity by 70-120% in isolated smooth muscle membranes. At 0.015 microM, nitrendipine, but not nifedipine, verapamil or diltiazem, also activated this enzyme. Nimodipine stimulated this Na+, K+ATPase three times more than nitrendipine at 15 nM. Marked stimulation of Na+,K+-ATPase by nimodipine was seen in membranes from rat and guinea pig aorta and rat vas deferens, but not in membranes from guinea pig heart or brain. Although it is not known whether these results are applicable to intact cells, the results are consistent with the hypothesis that vasodilation produced by nimodipine and nitrendipine may be due not only to inhibition of Ca2+ entry but also to the stimulation of the Na+ pump.

Activation of potassium transport induced by secretagogues in superfused submaxillary gland segments of rat and mouse

In order to investigate the actions of acetylcholine (ACh), catecholamines and substance P on K transport in the submaxillary gland, measurements of net K flux to and from the gland tissue using flame photometry, Na efflux from the tissue using radioactive 22Na, and membrane potential and input resistance using micro-electrodes were carried out on isolated superfused segments of rat and mouse submaxillary glands. ACh (5.5 X 10(-8) to 5.5 X 10(-4) M), phenylephrine (5 X 10(-7) to 5 X 10(-4) M) or substance P (10(-9) to 10(-5) M) stimulation for 5 min induced a transient K release followed by a small K uptake after the cessation of stimulation. The K release was markedly enhanced by the simultaneous addition of ouabain (10(-3) M). On the other hand, isoprenaline (2.5 X 10(-9) to 2.5 X 10(-5) M) induced a transient K uptake without any preceding K release. The K uptake was completely blocked by the addition of ouabain. Noradrenaline induced only K uptake at a low concentration (3 X 10(-7) M), but induced transient K release followed by marked K uptake at higher concentrations (3 X 10(-6) to 3 X 10(-4) M). The K release induced by noradrenaline was suppressed by the addition of phentolamine (10(-5) M), while the K uptake was suppressed by propranolol (5 X 10(-6) M). The K release induced by ACh, phenylephrine, noradrenaline or substance P was severely reduced by Ca omission from the superfusing solution and restored by the re-admission of Ca. The isoprenaline- or noradrenaline-induced K uptake was, however, little affected by Ca omission. Application of isoprenaline (2.5 X 10(-6) M) induced an increase in 22 Na efflux. The increase in 22Na efflux was completely abolished in the presence of ouabain. Local application to the tissue bath of isoprenaline (4.7 X 10(-13) to 4.7 X 10(-12) mole) or noradrenaline (5.7 X 10(-12) to 5.7 X 10(-11) mole) in the presence of phentolamine (10(-5) M) induced membrane hyperpolarization without any appreciable change in input resistance. The hyperpolarization was abolished in the presence of ouabain (10(-3) M) or propranolol (5 X 10(-6) M) or in a K-free or low Na solution. Higher doses of both agonists, however, induced depolarization or biphasic responses (initial depolarization followed by hyperpolarization). The depolarizations were accompanied by a moderate reduction in input resistance. It is concluded that in the rat and mouse submaxillary gland acinar cells cholinergic, alpha-adrenergic or substance P stimulation causes K release (and perhaps Na uptake) resulting in activation of the Na-K pump, while beta-adrenergic receptor stimulation might directly activate the Na-K pump resulting in K uptake, or might cause Na uptake resulting in activation of the Na-K pump.

Na+-K+-ATPase in rat vascular smooth muscle cell grown in vitro

This study has focused on the characteristics of the Na+-K+-ATPase in in vitro preparations of vascular smooth muscle cells (VSMCs) derived from the rat carotid artery. The maximum velocity of enzyme reaction (Vmax) for the specific activity of the enzyme in the VSMCs' preparations was 2.36 +/- 0.04 (SE) mumol Pi X mg cell protein-1 X h-1 or 0.82 +/- 0.02 mumol Pi X 10(6) cells-1 X h-1. The activation of the enzyme by potassium, sodium and ATP has been investigated. The half-maximal values for potassium and sodium activation of the enzyme in the preparations were 1.18 and 10-20 meq/l, respectively. The respective Vmax values for potassium and sodium activation were reached at concentrations of 4-10 and 80-100 meq/l. The Michaelis constant for ATP was 0.83 mM. Calcium exerted a potent inhibition on the activity of the enzyme (I50 at 1 mM). It has been concluded that the Na+-K+-ATPase kinetic pattern in in vitro preparations of VSMCs is quite similar to that observed in homogenates or subcellular fractions of other tissues.

Analysis of angiotensin-stimulated sodium transport in cultured smooth muscle cells from rat aorta

Angiotensin peptides (AI, AII, AIII) increased the rate of Na+ accumulation by smooth muscle cells (SMC) cultured from rat aorta. The stimulatory effect of AII on Na+ uptake was observed when Na+ exodus via the Na+/K+ pump was blocked either by ouabain or by the removal of extracellular K+. AII was at least ten times more potent than AIII and about 100 times more potent than AI in stimulating Na+ uptake. Saralasin had little effect on Na+ uptake by itself but almost completely blocked the increase caused by AII. The stimulation of net Na+ entry by AI, but not AII, was prevented by protease inhibitors. The stimulation of Na+ uptake was almost completely blocked by amiloride. Tetrodotoxin, which prevented veratridine from increasing Na+ uptake, had no effect on the response to AII. Angiotensin increased the rate of ouabain-sensitive 86Rb+ uptake (Na+/K+ pump activity) but had no effect on ouabain-sensitive ATPase activity in frozen-thawed SMC or in microsomal membranes isolated from cultured SMC. The stimulation of ouabain-sensitive 86Rb+ uptake by AII was blocked by saralasin. Omitting Na+ from the external medium prevented AII from increasing 86Rb+ uptake. AII had no effect on cell volume or cyclic AMP levels in the cultured SMC. These results suggest that angiotensin peptides activate an amiloride-sensitive Na+ transporter which supplies the Na+/K+ pump with more Na+, its rate-limiting substrate.

Pathogenesis of rheumatoid arthritis: a vascular hypothesis

Past research into the pathogenesis of RA has generally concerned itself with established inflammation. The present review summarizes alterations in microvascular anatomy and function which occur during the hypoxic state, in various experimental and disease conditions. It further shows that tissue hypoxia is a common finding in RA and that the microvascular alterations of RA are similar to those produced by experimental hypoxia. The available data suggest that microcirculatory compromise, concomitant with an increase in metabolic needs of synovial tissue, may initiate tissue injury via anoxia and acidosis, resulting in hydrolytic enzyme release, increased vascular permeability and acceleration of inflammatory processes. It is further believed that the microcirculatory abnormality may be generalized, accounting for the systemic manifestations often seen in RA. Factors effecting arteriolar blood flow obstruction are reviewed to identify areas for future investigation in RA and other disorders involving microvasculopathy. The multitude of longknown and newly recognized factors predisposing to vasospasm and vasodilatation have been outlined as a guide to possible mechanisms which may be operative in RA. An attempt has been made to gather and synthesize the available data in the hope that it may stimulate other investigators to pursue more definitive research into specific areas which may show early microvascular abnormalities in the pathophysiology of RA. Identification of factors operative early in the pathogenesis of RA, before it becomes self-perpetuating, may well be a step in the direction of preventing the ravages of this disease, or providing insight to more effective control.

Mechanism of extraneuronal adrenaline release induced by K+-free medium and ouabain, but not by high KCl in vas deferens of guinea-pig and rat

The effects of K+-free medium, ouabain or high KCl on the contractile response and spontaneous efflux of adrenaline were analyzed in denervated guinea-pig or rat vasa deferentia preloaded with 3 x 10(-5) M (+/-)-adrenaline. The K+-free medium-induced contraction of guinea-pig tissues was related to the concentration of adrenaline used during preloading and was suppressed by the addition of 2.5 mM Rb+ but not by 2.5 mM Cs+. The contractile effects of K+-free medium and 10(-5) M ouabain, but not the response to high KCl, were prevented by 5 x 10(-6) M phentolamine or 3 x 10(-5) M deoxycorticosterone. These contractions, therefore, appear to be mediated by adrenaline released from the extraneuronal compartment. The K+-free medium- or ouabain-induced contractions of denervated rat vasa deferentia were small in comparison with those of the guinea-pig tissue. The amount of spontaneous efflux of adrenaline from the guinea-pig preparation was significantly enhanced by omission of K+ or by 10(-4) M ouabain but not by 80 mM KCl. K+-free medium and 3 x 10(-5) M ouabain merely elicited a small membrane depolarization accompanied by a slight decrease of membrane resistance in the guinea-pig vas deferens, whereas 30 mM KCl markedly altered these electrical parameters. From these findings, it is suggested that the rapid increase in extraneuronal amine release following inhibition of Na+-K+-ATPase may be due to some direct effect of K+ deficiency or of ouabain on the binding between the enzyme and the catecholamine rather than to alterations of membrane properties as a consequence of enzyme inhibition.

Prostaglandins and ischemic heart disease

There is an abundance of information suggesting that prostaglandins are involved in the development and clinical expression of atherosclerosis. Many studies demonstrate a relationship between prostaglandins and the risk factors for peripheral and coronary artery disease. Thus, part of the mechanism by which hyperlipidemia, diabetes mellitus, smoking, hypertension, sex hormones, age, heredity, emotional stress and diet contribute to the development and progression of atherosclerosis may be through an imbalance between thromboxane A2 and prostaglandin I2. Recent studies show a temporal relationship between acute ischemic events (specifically, unstable angina) and a transcardiac increase in thromboxane B2, while others demonstrate a salutary effect of disaggregatory and vasodilatory prostaglandins in such patients. If prostaglandins and thromboxane prove important in ischemic vascular disease, attention will be directed at the correction of their pathologic imbalance. This may be accomplished by dietary manipulation as well as by the development of prostaglandin receptor antagonists or inhibitors of specific prostaglandin pathways.

Zero extracellular K+ and prostaglandin E release in the guinea-pig taenia coli

Prostaglandin E release rates from isolated strips of guinea-pig taenia coli increased during exposure to zero K+ bathing fluid, from control values of 0.78 +/- 0.11 ng/g per min to levels as high as 29.2 ng/per min. Release rates increased for 40-50 min and then remained constant or fell despite progressive increases in intracellular sodium [Nai+] or fall in intracellular potassium [Ki+]. Readmittance of K+ to the bathing solution resulted in rapid reversal of elevated prostaglandin E release rates. [Nai+] and [Ki+] were markedly more abnormal in strips exposed to zero K+ for 70-201 min compared to 30-min exposures. Upon the readdition of K+ after long zero K+ exposure, the rate of prostaglandin E release fell long before [Nai+] and [Ki+] returned to control levels. After K+ was readded to the bathing solution, the ion concentration of tissues exposed to zero K+ for 30 min returned to normal much more quickly than did those of tissues exposed for the longer time periods, yet the exponential rate constants for fall of prostaglandin E release rate after K+ was added were not significantly different after short or long zero K+ exposure. Thus there was a dissociation between the return of [Nai+] and [Ki+] and the fall of prostaglandin E release rate to control levels. Ouabain augmented prostaglandin E release under conditions where [Ki+] could not fall. Addition of known neurotransmitters present in this tissue to the bathing fluid did not augment prostaglandin E release. Guinea-pig taenia coli strips that had been incubated with [3H]arachidonic acid, constantly released [3H]arachidonic acid and [3H]prostaglandin E and a prostaglandin which cochromatographed with prostaglandin E but could not be converted to prostaglandin B by alkali and was shown to be 6-ketoprostaglandin F1 alpha. Release of [3H]arachidonic acid and [3H]prostaglandin E plus 6-[3H]ketoprostaglandin F1 alpha was increased when strips were exposed to zero K+. Data obtained in this study suggest the augmented prostaglandin E release seen during zero K+ or ouabain is related to increased availability of unbound arachidonic acid at the site of cyclooxygenase in the cell. Augmented prostaglandin E release is apparently not related to alterations in intracellular electrolyte concentrations or release of known neurotransmitters.

Effects on fluid balance induced by non-febrile intracerebroventricular infusions of PGE2, PGF2alpha, and arachidonic acid in the goat

Prostaglandins (PG) E2, F2 alpha (30 ng/kg . min-1) and arachidonic acid (150 and 300 ng/kg . min-1) were infused for 30 min into the lateral cerebral ventricle of conscious hydrated and non-hydrated goats. Like previously shown as concerns PGE1 PGE2 was found to inhibit the water diuresis and cause some increase in the renal sodium excretion in the hydrated animal, and to elicit thirst in the non-hydrated goat. The effects of PGE2 were enhanced when hypertonic (0.25 M) NaCl was simultaneously infused into the ventricle. The antidiuretic effect of PGF2 alpha was less pronounced, and drinking only occasionally occurred when this PG was infused into the non-hydrated animal. Only a weak, post-infusion reduction of the water diuresis was observed when arachidonic acid was administered into the hydrated goat. Neither the PG:s, nor arachidonic acid affected the temperature regulation of the animals. The possibility is discussed that the lack of febrile response was due to the choice of platinum-iridium as material for the cerebral implantations. It is suggested that PGE2 might have interacted with CSF Na+ in stimulating juxtaventricular receptors involved in the control of fluid balance. The experiments do not support the concept that PG:s of the E series constitute a cerebral humoral link in pyrogen-induced fever.

Calcium-dependent inhibition by prostaglandin E1 of spontaneous acetylcholine release from frog motor nerve

PGE1 (10(-7) M) in the absence of Ca2+ or at low external Ca2+ concentrations (0.2-0.5 mM) depressed the frequency of miniature end-plate potentials (m.e.p.p.s) in frog sartorius muscle, but m.e.p.p. amplitudes were unchanged. Both an increase in Ca2+ concentration to 2 mM or a blockade of Ca2+ uptake into mitochondria by metabolic inhibitors (5 X 10(-5) M 2,4-dinitrophenol or 2 X 10(-5) M rotenone) prevented the inhibitory action of PGE1 on m.e.p.p. frequency. We suggest that PGE1 may inhibit acetylcholine release from motor nerve terminals by promoting the active uptake of Ca2+ by mitochondria, or by facilitating the efflux of Ca2+ from the axoplasm into the extracellular medium.

Effects of cyclic AMP and dibutyryl cyclic AMP on cerebral hemodynamics and metabolism in the baboon

Adenosine 3',5'-cycle monophosphate (cyclic AMP) (0.5 mg/kg) was infused into the carotid artery of baboons anesthesized with sodium pentobarbital, causing a biphasic increase in cerebral blood flow (CBF) and reduction in cerebrovascular resistance (CVR) associated in each phase with stimulation of cerebral metabolism evidenced by increased cerebral oxygen consumption (COMRO2) and cerebral glucose consumption (CMRG1). Intracarotid dibutyryl cyclic AMP (0.5 mg/kg) caused a monophasic increase in CBF and reduction of CVR but failed to alter cerebral metabolism. This may be due to its rapid removal from the circulation with ineffective passage across the blood-brain barrier since intracisternal infusion of dibutyryl cyclic AMP caused sustained increase in CBF, CMRO2 and CMRG1 and reduction in CVR. Intracarotid AMP (0.4 mg/kg) and adenosine (0.3 mg/kg) caused an immediate and more marked increase in CBF and decrease in CVR unassociated with cerebral metabolic change making it unlikely that the observed effects of cyclic AMP can be attributed to its breakdown products. Cyclic AMP or its dibutyryl derivative may alter cerebral metabolism secondary to neuronal activation but increase in glucose/oxygen utilization ratio after intracarotid cyclic AMP and intracisternal dibutytyl cyclic AMP also suggests an influence on enzymatic regulation of glucose metabolism.

Influence of prostaglandin E1 on cerebral mechanisms involved in the control of fluid balance

The effects of infusions of PGE1 (30 ng/kg min-1) into the lateral cerebral ventricle were studied in the conscious, hydrated goat. The infusions caused release of antidiuretic hormone and increased renal sodium excretion. When PGE1 was infused together with hypertonic NaCl these effects became markedly enhanced and the infusion also induced drinking and a rise in the arterial blood pressure. Much weaker effects were obtained by the infusion of the hypertonic NaCl alone. This sodium-PGE1 interaction is discussed in relation to previously observed, central sodium-angiotensin II interaction. A more pronounced drinking effect was obtained in response to the intraventricular infusion of PGE1 + angiotensin II, than to the infusion of either substance separately. The PGE1 administered into the lateral cerebral ventricle did not induce any febrile response.