Concentration-dependent Increases In Tension Research Articles

Hypothalamic Na+,K(+)-ATPase inhibitor constricts pulmonary arteries of spontaneously hypertensive rats.

Hypothalamic inhibitory factor (HIF) is an endogenous high-affinity inhibitor of Na+,K(+)-ATPase with ouabain-like properties and has been implicated in the pathogenesis of genetic systemic hypertension. We wondered whether HIF might also be associated with the recently demonstrated pulmonary hypertension of spontaneously hypertensive rats (SHRs). We compared HIF effects on the contractility of isolated 2- to 3-mm pulmonary artery (PA) rings from SHRs and age-matched normotensive Sprague-Dawley (SD) rats. HIF caused a reversible, concentration-dependent increase in tension in PA rings of SHR and SD rats, whereas ouabain did not. PA tension development with HIF (4 nM final concentration) was significantly higher in SHRs than in SD rats: 308 +/- 56 mg (mean +/- SE) vs. 137 +/- 26, respectively, p < 0.05. Abdominal aortic contractions induced by HIF did not differ between SHRs and SD rats. In SHRs, but not SD rats, the effect on PA rings was significantly greater than on aortic rings. In all cases, contraction was abolished by phentolamine but was unaffected by calcium-channel blockade using verapamil. HIF-induced tension development required external Ca2+. We conclude that PA rings from SHRs are more sensitive to Na+,K(+)-ATPase inhibitory effects of HIF than PA rings from SD rats, which may contribute to the observed pulmonary hypertension in SHR. Local modulation of the Na+,K(+)-ATPase-adrenergic neuroeffector interaction may be the vasoconstrictive mechanism of action of HIF in these vessels.

Modulation of NO and endothelin by chronic increases in blood flow in canine femoral arteries.

Experiments were designed to determine whether chronic increases in arterial blood flow alter production of or response to nitric oxide and endothelin. Canine femoral arteries proximal to an arteriovenous fistula- and from the contralateral sham-operated blood vessels were removed, cut into rings, and suspended for measurement of isometric force in organ chambers. The remainder of the artery was homogenized for measurement of endothelin content by radioimmunoassay. NG-monomethyl-L-arginine (10(-4) M) caused concentration-dependent increases in tension only in fistula-operated arteries. Endothelium-dependent relaxations to acetylcholine and BHT-920 were greater in fistula- compared with sham-operated arteries. These differences were reduced by the arginine analogue. Pertussis toxin (100 ng/ml) inhibited relaxations to acetylcholine only in fistula-operated arteries and to BHT-920 only in sham-operated arteries. Contractions to endothelin-1 were greater in fistula- compared with sham-operated arteries. These results suggest that chronic increases in blood flow enhance the tonic and receptor-stimulated production of nitric oxide and its release by receptors coupled to pertussis toxin-sensitive guanine nucleotide regulatory proteins. Furthermore, chronic increases in blood flow may either inhibit the production of endothelin or promote its depletion from endothelial cells while simultaneously increasing the sensitivity of the smooth muscle to its contractile effects.

Pacing-Induced Heart Failure in the Dog: Evaluation of Peripheral Vascular α-Adrenoceptor Subtypes

Postjunctional alpha-adrenoceptor characteristics were evaluated in canine dorsal pedal arterial and saphenous vein rings studied before and after development of severe pacing-induced heart failure (CHF). Before CHF, all agonists produced concentration-dependent increases in tension of both blood vessels. After development of CHF, the responsiveness and sensitivity of the vessels to the alpha 1-agonists and the mixed agonists were significantly increased as compared with control. The maximum responses to BHT 920 and BHT 933 remained unaltered after CHF, but both vessels showed decreased sensitivity to BHT 920. Before CHF, the rank order of potency with respect to norepinephrine (NE) for the dorsal pedal artery was as follows: NE greater than epinephrine greater than methoxamine greater than BHT 933 greater than BHT 920, and for the saphenous vein was epinephrine greater than NE greater than BHT 933 greater than methoxamine greater than BHT 920. At peak CHF, the rank order of potency for the artery was epinephrine greater than NE greater than methoxamine greater than BHT 933 greater than BHT 920, whereas in the vein BHT 920 was approximately 80 times less potent than NE (as compared with being only five times less potent before CHF). Prazosin was a potent, competitive antagonist (pA2 values of 9.2 and 9.0 for the artery and the vein) of methoxamine-induced contractions before development of CHF. Prazosin had a 10-fold lower potency against epinephrine-induced contractions in the dorsal pedal artery, whereas it was not competitive against epinephrine in the saphenous vein. Against the selective alpha 2-agonists, prazosin either showed no antagonism or was not competitive. After CHF, prazosin was non competitive against all agonists tested. Yohimbine was a potent, competitive antagonist against BHT 920 both before and at CHF. Yohimbine had intermediate antagonism against epinephrine and produced no antagonism of methoxamine-induced contractions. We conclude that increased reactivity and sensitivity of the peripheral vasculature to alpha 1-agonists occurs at CHF.

Inositol 1,4,5-trisphosphate releases intracellular Ca2+ in permeabilized chick atria

Inositol 1,4,5-trisphosphate (IP3) and caffeine evoked transient, reversible, and concentration-dependent increases in tension in saponin-treated chick atrial muscle. Contractures evoked by IP3 and caffeine were detected in solutions with 70 microM EGTA at pCa 7.0. In the presence of 7 mM EGTA, neither IP3 nor caffeine was able to evoke a contracture. Maximally effective concentrations of IP3 (20 microM) and caffeine (20 mM) developed tensions to approximately 44 and 83% of that elicited by pCa 5.0 (maximum tension = 100%), respectively. The IP3- or caffeine-induced contractures were consistently reproduced when the sarcoplasmic reticulum (SR) had previously been loaded with calcium. Preexposure to caffeine suppressed the following IP3-induced response. When ryanodine (1-10 microM) was present throughout the SR-loading cycle, the responses to IP3 and caffeine were prevented. However, when ryanodine was added after the SR was loaded with calcium, neither the response to IP3 nor that to caffeine was affected. These results are consistent with the hypothesis that ryanodine inhibition requires prior activation of the SR calcium-release channel. It is concluded that both IP3 and caffeine increased tension in the SR by releasing calcium from it. The effect of IP3 is consistent with its messenger role as a calcium-mobilizing agent.

Effects of EDCF and endothelin on phosphatidylinositol hydrolysis and contraction in rat aorta

Endothelium-derived constricting factor (EDCF) and endothelin are peptidergic substances produced and released from endothelial cells that induce contraction of vascular smooth muscle. The purpose of the present study was to investigate possible mechanisms by which EDCF and endothelin elicit contraction. Exposure of rat aorta to EDCF or synthetic endothelin resulted in time- and concentration-dependent increases in tension and levels of inositol monophosphate, a breakdown product of the phosphatidylinositides. A 10-s exposure to endothelin elevated levels of inositol 1,4,5-trisphosphate. Trypsinization or heating of EDCF prevented the contraction and inositol monophosphate formation. To assess whether EDCF and endothelin may act as endogenous agonists of the dihydropyridine-sensitive Ca2+ channel, we evaluated the ability of the dihydropyridine Ca2+ channel agonist (+)-S202-791 to increase the formation of the inositol phosphates. (+)-S202-791 increased inositol monophosphate formation. However, in contrast to that elicited by EDCF and endothelin, the increase in inositol monophosphate because of (+)-S202-791 was abolished by pretreatment with the cyclooxygenase inhibitor indomethacin (10 microM). These results suggest that contractions induced by EDCFs may be mediated through activation of phospholipase C and subsequent production of second messengers.

Differential sensitivity to endothelin in canine arteries and veins.

Experiments were designed to compare the sensitivity of venous and arterial smooth muscle to endothelin and to determine whether contractions to the peptide could be inhibited by endothelium-derived relaxing factor and nitric oxide. Rings of canine left anterior descending coronary, femoral, and mesenteric arteries and femoral and saphenous veins with and without endothelium were suspended for measurement of isometric force. In the presence of indomethacin, phentolamine, and propranolol, endothelin initiated concentration-dependent increases in tension in all rings. The veins were more sensitive to the peptide than were the arteries. Endothelin depolarized the smooth muscle of the saphenous veins and mesenteric arteries; the threshold concentration for depolarization was approximately 100 times lower in the veins (10(-10) M) than in the arteries (10(-8) M). Removal of the endothelium enhanced the sensitivity only of venous smooth muscle to endothelin. However, stimulation of the endothelium in the arteries with either acetylcholine or the calcium ionophore A23187 rapidly inhibited the maximal tension developed to the peptide. Nitric oxide inhibited contractions to endothelin in arteries and veins without endothelium; the inhibition was greater in the arteries than in the veins. These results indicate that venous smooth muscle is more sensitive than arterial smooth muscle to endothelin. In both blood vessels, endothelium-derived relaxing factor(s) can inhibit contractions to the peptide.

Calcium retention and increased vascular reactivity caused by a hypothalamic sodium transport inhibitor.

1. Using a previously established method of isolating an active-sodium-transport inhibitor (ASTI) from hypothalamic cell culture medium, the inhibitor was isolated and partially purified from sequential passages through Sephadex G-25 and h.p.l.c., and its effects on de-endothelialized rabbit aortic strips were investigated. 2. ASTI caused a cumulative concentration-dependent increase in tension which reversed slowly after wash, and the wash showed an identical effect on fresh strips. 3. Ouabain, used as a control, also caused a concentration-dependent increase in tension which reached a plateau at a concentration of 10 mmol/l. Both ouabain and ASTI caused a significant potentiation of the vasoconstrictor effect of noradrenaline at concentrations of 1 nmol/l-0.1 mmol/l. 4. Both ASTI and ouabain caused a significantly greater (P less than 0.01) calcium retention than control medium in aortic strips. 5. Incubation of ASTI with prolidase, chymotrypsin and carboxypeptidase A destroyed the vasoconstrictor effects as well as its inhibitory effects on sodium, potassium-dependent adenosine triphosphatase and sodium efflux from erythrocytes, but leucine aminopeptidase was ineffective. 6. These studies suggest that hypothalamic cells in culture release a peptidic inhibitor of active sodium transport which increases vascular reactivity, potentiates vasoconstrictor effects of noradrenaline and causes calcium retention.

Effects of leukotriene D4 on the mechanical and electrical properties of guinea-pig isolated trachealis.

1. The effects of leukotriene D4 (LTD4) on mechanical and electrical activity were examined in guinea-pig isolated trachealis muscle and compared with two other bronchoconstrictors, methacholine and potassium chloride (KCl). 2. LTD4 elicited concentration-dependent increases in tension in trachealis muscle which were slower in time course than responses induced by either methacholine or KCl. The maximum response to LTD4 was approximately 85% of the methacholine maximum. 3. At a concentration close to the EC50 for tension changes, LTD4 had no significant effect on either transmembrane potential or slow wave activity recorded in single trachealis cells. 4. At a concentration close to the EC90 for tension changes, LTD4 caused significant membrane depolarization, transiently reduced the amplitude and increased the frequency of slow wave discharge and ultimately abolished slow wave discharge. LTD4-induced depolarization was less marked, and developed more slowly, than that evoked by either methacholine or KCl. 5. These results show that LTD4 can elicit substantial increases in tension without altering transmembrane potential and are consistent with the view that LTD4 initiates contraction mainly through potential-independent mechanisms. However, at high concentrations the depolarization evoked by LTD4 allows the possibility that potential-dependent mechanisms may contribute to the spasm.

The effect of forskolin on the isometric contraction of the isolated hemidiaphragm of the rat.

The effects of forskolin on the parameters of the isometric contraction alone and in combination with aminophylline and isoprenaline were studied on the isolated hemidiaphragm of the rat during direct electrical stimulation. Forskolin (2.6 - 18.2 mumol l-1) produced a concentration-dependent increase in tension developed (Td) and, to a lesser extent, in the maximum rate of rise in tension (dT/dt max). The dose-response curve for the action of forskolin (2.6 - 18.2 mumol l-1) on Td was shifted to the left in the presence of a standard concentration of aminophylline (0.32 mmol l-1). Forskolin (5.2 mumol l-1) produced a further and significant increase in both Td and dT/dt max in the presence of isoprenaline (0.24 mumol l-1) in the bath. In a calcium-free medium, the effects of forskolin (7.80 and 18.2 mumol l-1) on Td and dT/dt max were significantly weaker than in a medium containing calcium. These data indicate that forskolin increases the isometric contraction of the isolated hemidiaphragm probably by activating the adenosine 3':5'-cyclic monophosphate (cyclic AMP) generating system. These effects are possible only in the presence of calcium.

Endothelium-dependent contractions to arachidonic acid are mediated by products of cyclooxygenase.

Arachidonic acid produces endothelium-dependent relaxation in canine arteries and endothelium-dependent contraction in veins. In canine femoral arteries, the relaxation is prevented by inhibitors of cyclooxygenase. To determine the role of cyclooxygenase in the contraction evoked by arachidonic acid in the veins, rings of canine femoral and intrapulmonary veins, with and without endothelium, were suspended in organ chambers and set at their optimum length for isometric tension measurements. In rings of femoral and pulmonary vein contracted with norepinephrine, arachidonic acid produced a concentration-dependent increase in tension that was eliminated by removal of the endothelium or by treatment with the inhibitors of cyclooxygenase (indomethacin, meclofenamate, or acetylsalicyclic acid). The contractions were not prevented by inhibitors of thromboxane synthetase or prostacyclin synthetase or lipoxygenase. Pulmonary and femoral veins with or without endothelium relaxed to low, but contracted to high concentrations of prostacyclin and prostaglandin E2. Prostaglandin F2 alpha caused endothelium-independent contractions in both blood vessels. The present study suggests that the endothelium-dependent contractions to arachidonic acid observed in canine veins are mediated by prostanoids other than thromboxane and prostacyclin.

Lack of effect of leukotriene D4 on Ca-uptake in airway smooth muscle.

The effects of verapamil on leukotriene D4 (LTD4)- and KCl-induced contractions and 45Ca-uptake were examined in guinea-pig isolated tracheal smooth muscle. Both LTD4 (0.1 to 200 nmol l-1) and KCl (8 to 125 mmol l-1) produced concentration-dependent increases in tension in the tracheal preparations. Verapamil (1 mumol l-1) inhibited the tension responses induced by both LTD4 and KCl. LTD4 failed to increase the lanthanum-resistant Ca content of tracheal smooth muscle at either low (EC25; 3 nmol l-1) or high (EC90; 50 nmol l-1) concentrations. Verapamil did not modify this result. KCl (90 mmol l-1) increased the lanthanum-resistant Ca content of the smooth muscle by approximately 60% over basal levels. This effect was completely inhibited by verapamil (1 mumol l-1). It is concluded that in this tissue, LTD4 utilizes principally an intracellular source of Ca2+ to initiate contraction whereas KCl is dependent upon the uptake of Ca2+ from the extracellular compartment. It is suggested that the inhibitory effects of verapamil may reflect an intracellular mechanism of action directed against Ca2+ release initiated by LTD4.

Facilitating interaction between rauwolscine and angiotensin in the mesenteric artery of the rabbit.

The interaction between rauwolscine and angiotensin II was investigated in the isolated mesenteric artery of the rabbit. Rauwolscine, known as an antagonist at alpha 2-adrenoceptors, did not induce contraction itself but interacted with angiotensin to produce a facilitated response of the vascular tissue. In the presence of rauwolscine, the contractile response of the tissue to angiotensin was markedly enhanced. The degree of facilitation appeared to be dependent on the rauwolscine concentration used rather than that of angiotensin. Moreover, rauwolscine induced a concentration-dependent increase in tension (pD2 = 6.8) in the presence of even subcontractile concentrations of angiotensin (10(-10) mol/l). This effect was not attributable to an indirect action involving presynaptic catecholamines, as revealed by the use of tissue strips from animals pretreated with reserpine or after chemical sympathectomy. Furthermore, an interaction via the prostaglandin system was excluded by negative results obtained with indomethacin. The 'agonistic effect' of rauwolscine was significantly attenuated by phentolamine (alpha 1/alpha 2) but not by prazosin (alpha 1) or phenoxybenzamine when applied for only a short time. The alpha 2-antagonist BDF 6143 behaved like rauwolscine whereas the alpha 1-antagonist corynanthine, a stereoisomer of rauwolscine, did not. The results indicate that the 'rauwolscine effect' is mediated by a receptor with alpha 2-characteristics. In general, angiotensin appears to interfere with some process which determines the expression of a drug's intrinsic effect.