Different and common intracellular calcium-stores mobilized by noradrenaline and caffeine in vascular smooth muscle

Vasorelaxant effects of 2-nitro-1-phenyl-1-propanol in rat aorta

2-Nitro-1-phenyl-1-propanol (NPP) is a nitro alcohol that is known as an intermediate in the synthesis of sympathomimetic agents, such as norephedrine. The present study investigated the vasoactive effects of NPP on rat aorta. In endothelium-intact aortic rings, NPP fully relaxed contractions that were induced by phenylephrine, KCl, and U-46619. The relaxant effects of NPP on phenylephrine-elicited contractions remained unaffected by NG-nitro-l-arginine methyl ester (l-NAME), indomethacin, propranolol, tetraethylammonium, 4-aminopyridine, and glibenclamide. Conversely, pretreatment with 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), cis-N-(2-phenylcyclopentyl)-azacyclotridec-1-en-2-amine hydrochloride (MDL-12,330A), and N-[2-(P-bromocinnamylamino)ethyl]-5-isoquinolinesulfonamide dihydrochloride (H-89) reduced the ability of NPP to relax contractions that were elicited by phenylephrine. NPP inhibited the vasoconstrictor response that was induced by Ca²⁺ in aortic rings that were stimulated by pharmacomechanical or electromechanical coupling with phenylephrine and 60 mmol/L KCl, respectively, and after the depletion of intracellular Ca²⁺ stores. Such effects of NPP were significantly reversed by pretreatment with the guanylyl cyclase inhibitor ODQ and weakly influenced by the adenylyl cyclase inhibitor MDL-12,330A. In Ca²⁺ -free medium, NPP inhibited transient contractions that were induced by phenylephrine but not caffeine. In homogenates of aortic rings, NPP increased cyclic guanosine 3',5'-monophosphate (cGMP) and cyclic adenosine 3'-5'-monophosphate levels, but this effect was statistically significant only for cGMP. In conclusion, in contrast to the vasoconstrictor amine norephedrine, NPP is a vasodilator in rat aorta, and its relaxant effects are likely attributable to cGMP production.

The vasorelaxant effects of 1-nitro-2-phenylethane involve stimulation of the soluble guanylate cyclase-cGMP pathway

1-Nitro-2-phenylethane is the first organic NO₂-containing molecule isolated from plants. It possesses interesting hypotensive, bradycardic, and vasodilator properties, but the mode by which it induces vasorelaxation is still unknown. The underlying mechanism involved in the vasodilator effect of 1-nitro-2-phenylethane was investigated in rat aorta. The vasorelaxant effects of 1-nitro-2-phenylethane did not depend on endothelial layer integrity, and the effects were refractory to L-N(G)-nitroarginine methyl ester (L-NAME)-induced nitric oxide synthase inhibition. Vasorelaxation was similarly resistant to treatment with indomethacin, cis-N-(2-phenylcyclopentyl)-azacyclotridec-1-en-2-amine hydrochloride (MDL-12330A), and KT5720, indicating that neither prostaglandin release nor adenylyl cyclase activation is involved. Conversely, methylene blue- and ODQ-induced guanylate cyclase inhibition reduced the vasorelaxation induced by 1-nitro-2-phenylethane. The pharmacological blockade of K(+) channels with tetraethylammonium, glybenclamide, and 4-aminopyridine also blunted vasorelaxation induced by 1-nitro-2-phenylethane. The effects of 1-nitro-2-phenylethane were reversed by 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) and comparable to the effects induced by sodium nitroprusside. In silico analysis using an Ns H-NOX subunit of guanylate cyclase revealed a pocket on the macromolecule surface where 1-nitro-2-phenylethane preferentially docked. In vitro, 1-nitro-2-phenylethane increased cyclic guanosine 3',5'-monophosphate (cGMP) levels in rat aortic rings, an effect also reversed by ODQ. In conclusion, 1-nitro-2-phenylethane produces vasodilator effects by stimulating the soluble guanylate cyclase-cGMP pathway.

Relaxant mechanisms of 3, 5, 7, 3', 4'-pentamethoxyflavone on isolated human cavernosum

We have investigated effects and mechanisms responsible for the activity of 3, 5, 7, 3', 4'-pentamethoxyflavone (PMF) on isolated human cavernosum. PMF is the major flavone isolated from Kaempferia parviflora claimed to act as an aphrodisiac. PMF caused relaxation of phenylephrine precontracted human cavernosal strips, and this effect was slightly inhibited by N(G)-nitro-l-arginine, a nitric oxide synthase inhibitor, but not by ODQ (soluble guanylate cyclase inhibitor), TEA (tetraethylammonium, blocker of voltage-dependent K(+) channels) or glybenclamide (blocker of ATP-dependent K(+) channels). PMF did not significantly inhibit the relaxant activity of glyceryltrinitrate or acetylcholine on human cavernosal strips precontracted with phenylephrine. In contrast, sildenafil (phosphodiesterase inhibitor) potentiated the relaxant activity of glyceryl trinitrate but not of acetylcholine. In normal Krebs solution with nifedipine (blocker of l-type Ca(2+) channels), or in Ca(2+)-free Krebs solution, PMF caused a further inhibition of human cavernosum contracted with phenylephrine. In human cavernosum treated with thapsigargin (inhibitor of sarcoplasmic reticulum Ca(2+)-ATPase) in Ca(2+)-free medium, PMF suppressed the concentration-response curve of human cavernosum to phenylephrine and a further suppression was found when SKF-96365 (a blocker of store-operated Ca(2+) channels and Y-27632 (inhibitor of Rho-kinase)), but not nifedipine, were added sequentially. Thus, PMF had only a weak effect on the release of nitric oxide, and had no effect as a K(ATP)- or K(Ca) channel opener, a phosphodiesterase inhibitor, a store-operated Ca(2+) channel blocker or a Rho-kinase inhibitor. Therefore, these studies suggest that PMF causes relaxation of human cavernosum through voltage-dependent Ca(2+) channels and other mechanisms associated with calcium mobilization.

Hypotensive activity of an n-butanol extract and their purified compounds from leaves of Phyllanthus acidus (L.) Skeels in rats

We aimed to investigate the effects, identify the active substances and establish the mechanisms involved in the hypotensive activity of an n-butanol extract from leaves of Phyllanthus acidus (PA extract). PA extract caused a decrease in blood pressure of anesthetized rats that was not modified by atropine or propranolol. PA extract caused a persistent dilatation of thoracic aortic rings preconstricted with either phenylephrine or KCl, and these effects were not modified by LNA or removal of the vascular endothelium. For phenylephrine-preconstricted aortic rings, the dilatory activity of the PA extract was not modified by atropine, propranolol or indomethacin. TEA, glybenclamide or ODQ significantly inhibited the dilatory activity of the PA extract on endothelium-denuded aortic rings. Nifedipine or a Ca(2+)-free medium depressed the aortic rings constrictor response to phenylephrine, and that was further augmented by the PA extract. Adenosine, 4-hydroxybenzoic acid, caffeic acid, hypogallic acid, and kaempferol were isolated from the PA extract. Each caused a decrease in blood pressure and dilatation of the aortic rings. LNA or removal of the endothelium reduced this activity. ODQ and TEA attenuated the vasodilatory activity of adenosine whereas glybenclamide and ODQ attenuated the effect of hypogallic acid. These results suggest that the hypotensive activities of the PA extract is likely the result of the direct action of these five compounds on the blood vessels by stimulating release of nitric oxide from the vascular endothelium, in part through stimulation of soluble guanylate cyclase, and opening of K(ATP) and K(Ca) channels in the vascular smooth muscle.

Biphasic effects of sodium danshensu on vessel function in isolated rat aorta

AIM

To investigate the effects of sodium danshensu on vessel function in isolated rat aortic ring.

METHODS

Thoracic aortae from normal rats were isolated and equilibrated in organ bath with Krebs-Henseleit buffer and ring tension was recorded. Effects of sodium danshensu on basal tonus of the vessel and its effects on vessel contraction and relaxation with or without endothelium were observed.

RESULTS

In thoracic arteries under basal tonus, sodium danshensu (0.3-3 g/L) produced a dose-dependent transient contraction. In phenylephrine-precontracted thoracic arteries with or without endothelium, low concentration (0.1-0.3 g/L) of sodium danshensu produced a weak contraction, while high concentrations (1-3 g/L) produced a pronounced vasodilator after a transient vasocontraction. Pre-incubation with sodium danshensu could inhibit vessel contraction induced by phenylephrine and potassium chloride in a concentration-dependent way. Sodium danshensu inhibited phenylephrine- and CaCl(2)-induced vasoconstriction in Ca(2+)-free medium. Pre-incubation with tetraethylammonium, a non-selective K(+) channel blocker, and apamin, a small-conductance calcium-activated K(+) channel blocker partially antagonized the relaxation response induced by sodium danshensu. However, iberiotoxin (big-conductance calcium-sensitive K(+) channel blocker), barium chloride (inward rectifier K(+) channel blocker), and glibencalmide (ATP-sensitive K(+) channel blocker) had no influence on the vasodilation effect of sodium danshensu.

CONCLUSION

Sodium danshensu showed a biphasic effects on vessel tension. While low dosage of sodium danshensu produced small contraction possibly through transient enhancement of Ca(2+) influx, high dosage produced significant vasodilation mainly through promoting the opening of non-selective K(+) channels and small-conductance calcium-sensitive K(+) channels in the vascular smooth muscle cells.

Effect of caffeine on response of rabbit isolated corpus cavernosum to high K+ solution, noradrenaline and transmural electrical stimulation

1. Caffeine has wide-ranging activities on smooth muscles, including contractile and relaxant effects. The aim of the present study was to examine the activity of caffeine on rabbit corpus cavernosum (RCC). 2. The effects of caffeine (0.5-4.0 mmol/L) on the response of RCC to high K+ solution, noradrenaline (NA) and transmural electrical stimulation (EFS) were studied in a tissue bath system. 3. Caffeine did not contract the RCC. However, 0.5-4.0 mmol/L caffeine caused concentration-dependent relaxation of tension development in high-K+ (120 mmol/L) solution in contrast with the solvent control. At 4.0 mmol/L caffeine, high-K+ solution-induced tone of the RCC was reduced by 73.4 +/- 7.3%. Caffeine (0.5-4.0 mmol/L) also concentration-dependently relaxed NA (12.5 micro mol/L)-induced tonic contraction of the RCC. At 4.0 mmol/L caffeine, NA-induced tone of the RCC was reduced by 41.1 +/- 7.0%. Incubation of RCC in 2.0 mmol/L caffeine for 30 min prior to EFS (1-40 Hz) caused a marked rightward shift in the frequency-response curve. 4. The results of the present study suggest that caffeine exhibits relaxant activity on rabbit cavernosal smooth muscle and the mechanism of this activity possibly involves inhibition of Ca2+ signalling.

Alterations by age of calcium handling in rat resistance arteries

This study investigated the alterations by age of Ca handling in rat small mesenteric arteries. The contractile responses to phenylephrine and caffeine in small mesenteric arteries from young and old rats were studied in Ca -containing and Ca -free medium. In Ca -containing medium the contraction to phenylephrine (100 micro M) but not to caffeine (10 mM) was greater in old than in young rats. Concentration-response curve to phenylephrine was affected to the same extent by nifedipine (1 micro M) in both age groups, whereas ryanodine (20 micro M) decreased the maximal response to phenylephrine only in young rats. These results suggest the participation of intracellular Ca handling on the observed differences by aging. In Ca -free medium, phenylephrine (10 micro M) but not caffeine (10 mM) induced a greater contraction in old than in young animals, corroborating the results obtained in Ca -containing solution. The greater response to phenylephrine observed in old rats cannot be explained by an increase in the inositol 1,4,5-trisphosphate (IP ) formation because the accumulation of inositol phosphates by phenylephrine was not affected by aging. Results obtained in Ca -free medium using caffeine after phenylephrine or vice versa suggest a common intracellular Ca pool. Pretreatment with ryanodine in Ca -free medium almost abolished contractile response to phenylephrine and caffeine in young rats but only partially decreased them in old animals, suggesting an impairment in the Ca -induced Ca release (CICR) mechanism leading to an increase in the stored Ca content. The greater amount of stored Ca could explain the higher contractile response to phenylephrine observed in aged rats. As a consequence of all these changes due to aging, an imbalance between the two Ca release mechanisms from sarcoplasmic reticulum was observed with a major role of Ca induced release by IP at the expense of an impairment of CICR mechanism. This observation will also help explain the results obtained in the presence of extracellular Ca, where phenylephrine induced a greater maximum response in old animals in spite of a decrease in the midrange sensitivity to this agonist.

Alpha-adrenoceptor interaction of tetrandrine and isotetrandrine in the rat: functional and binding assays

The action of 1S,1'S-tetrandrine, a bisbenzyltetrahydroisoquinoline alkaloid, on alpha1-adrenoceptors has been compared with that of its isomer 1R,1'S-isotetrandrine. The work includes binding assays to analyse the affinity of these products for the [3H]prazosin binding site of rat cerebral cortical membranes and functional studies on rat isolated aorta to examine the effects of both alkaloids on intracellular calcium processes related or not to alpha-adrenoceptor activation. A radioligand receptor-binding study showed that both compounds interacted with the alpha1-adrenoceptors displacing [3H]prazosin from the specific binding site. The Ki values (inhibition constants) were 0.69+/-0.12 and 1.6+/-0.4 microM for tetrandrine and isotetrandrine, respectively. The functional studies showed that both alkaloids concentration-dependently inhibited noradrenaline-induced contraction in Ca2+-free solution (IC50 values, i.e. the concentrations needed to induce 50% inhibition, were 252.8 and 174.9 microM for tetrandrine and isotetrandrine, respectively), the spontaneous contractile response elicited by extracellular calcium after depletion of noradrenaline-sensitive intracellular stores (increase in resting tone; IC50 values 11.6 and 19.6 microM for tetrandrine and isotetrandrine, respectively) and the refilling of intracellular Ca2+ stores sensitive to noradrenaline (IC50 values 7.4 and 14.9 microM for tetrandrine and isotetrandrine, respectively). The results show that tetrandrine and isotetrandrine interact with alpha1-adrenoceptors by displacing the [3H]prazosin binding site and that both compounds inhibit mainly the Ca2+-dependent process and have less action on alpha1-adrenoceptors. Tetrandrine is more potent than isotetrandrine.

Pyridazine derivatives XIV. Study of the vasorelaxant action of 6-aryl-5-piperidino-3-hydrazinopyridazines in isolated rat thoracic aorta: comparison with hydralazine

1. For several years we have been working on the synthesis of modified hydrazinopyridazines which have proved to possess remarkable vasorelaxant and antihypertensive activity. We now report the vasodilator effects of three novel 6-aryl-5-piperidino-3-hydrazinopyridazines (1a, 1b and 1c), structurally related to the well-known antihypertensive drug hydralazine. 2. Hydralazine and the new hydrazinopyridazines relaxed, in a concentration-dependent and nonspecific way, the contractions elicited by noradrenaline or a high K+ concentration in rat aortic rings with or without endothelium. According to the IC50 (50% inhibitory concentrations) values obtained, the vasorelaxant potency of the new compounds was greater than that of hydralazine. 3. In a Ca2+-free medium, the contractions provoked by noradrenaline or caffeine were significantly inhibited by the new hydrazinopyridazines and by hydralazine. 4. Hydralazine and the novel molecules did not significantly modify basal, noradrenaline- or K+-induced 45Ca2+ uptake. 5. These results suggest that 1a, 1b and 1c have an endothelium-independent vasorelaxant activity greater than that of hydralazine in isolated rat aortic rings, which seems not to be mediated by a blockade of transmembrane Ca2+ movements through specific channels. This effect could be due, at least in part, to an intracellular mechanism of action.

Changes of response to caffeine in Ca(2+)-deficient medium in guinea-pig tenia coli

1. After the washing out of the first caffeine, the second 25 mM caffeine-induced tension was about 50% of the first response in tenia coli. Cyclopiazonic acid (CPA), a specific sarcoplasmic reticulum Ca(2+)-ATPase blocker, almost inhibited the contraction to the next application of caffeine after the washing out caffeine in the presence of CPA. The peak tension to caffeine increased after the pretreatment with 10 mM K+ for 1 min. After pretreatment with K+ in the presence of 10(-7) M nifedipine (only phasic response), the tension to caffeine also increased. 2. The response to caffeine was rapidly reduced after incubation in a Ca(2+)-free medium and was abolished after 6 min of incubation. However, the response to caffeine in Ca(2+)-free medium 30 sec. after pretreatment with 10 mM K+ in normal Ca medium was larger. 3. These results suggest that, when the cell membrane is in the resting state, Ca2+ enters the cytoplasm by a leaky pathway and enters storage sites through Ca(2+)-ATPase. Furthermore, when the cell membrane is depolarized for a short period (less than 1 min), Ca2+ entry occurs, leading to refilling of the Ca2+ storage sites utilized by caffeine. The retention of Ca2+ in the storage sites in the Ca(2+)-free condition in tenia coli was less relative to the position in vascular smooth muscle.

Study of the in vivo and in vitro cardiovascular effects of a hydralazine-like vasodilator agent (HPS-10) in normotensive rats

1. In this work, the cardiovascular effects of HPS-10, a new vasodilator agent, were studied in rats. 2. In conscious normotensive rats, oral administration of HPS-10 (4-9 mg kg-1) produced a dose-related and long-lasting fall in systolic arterial blood pressure (ED30 of 5.32 mg kg-1), accompanied by an increase in heart rate (ED30 of 8.43 mg kg-1). This tachycardia was totally inhibited by pretreatment with (+/-)-propranolol (10 mg kg-1, p.o.). 3. In anaesthetized normotensive rats, HPS-10 (0.3-0.6 mg kg-1, i.v.) produced a gradual, dose-dependent and sustained decrease in systolic, diastolic and mean arterial pressure (MAP) (ED30 for MAP of 0.41 mg kg-1, i.v.), accompanied by a significant bradycardia at high doses (> 0.4 mg kg-1; ED20 of 0.61 mg kg-1, i.v.). HPS-10 (0.5 mg kg-1, i.v.) did not modify the positive chronotropic effects induced by intravenous administration of noradrenaline (NA; 5 micrograms kg-1), angiotensin II (AII; 0.2 microgram kg-1) and nicotine (200 micrograms kg-1) but markedly inhibited the hypertensive response produced by these agents. 4. In rat isolated rubbed aorta, HPS-10 (0.1-1 mM) non-competitively and with almost equal effectiveness antagonized the contractions induced by NA, AII (in normal Krebs solution) and Ca2+ (in depolarizing Ca(2+)-free high-K+ 50 mM solution). In the experiments in Ca(2+)-free medium, HPS-10 (1 mM) considerably inhibited the contractions induced by NA, AII and caffeine in rat aorta. 5. Furthermore, in the studies with radioactive Ca2+, HPS-10 (1 mM) did not modify the basal uptake of 45Ca2+ but strongly decreased the influx of 45Ca2+ induced by NA, AII and K+ in rat aortic rings. 6. In rat isolated atria, HPS-10 (1 mM) produced a positive inotropic/negative chronotropic effect. 7. HPS-10 (0.3 mM) significantly inhibited the sustained and transient Ba2+ inward current (IBa) recorded in whole-cell clamped rat aortic myocytes. 8. These results indicate that the non-selective vasorelaxant effects of HPS-10 in rat aortic rings can be attributed to transmembrane Ca(2+)-antagonist activity and an intracellular action on smooth muscle cells. The direct vasodilator action of HPS-10 observed in rat isolated aorta may be responsible for the HPS-10 hypotensive activity in anaesthetized normotensive rats.

Vasorelaxant properties of brefeldin A in rat aorta

The effects of brefeldin A, a putative specific agent that disassembles the Golgi apparatus were assessed on the contractility of de-endothelised rat aorta. Brefeldin A inhibited, either as pre- or as post-treatment, the contractions elicited by K+ (75 mM) or phenylephrine (10 microM), being significantly more potent upon the latter. The thapsigargin (1 microM)-induced rat aorta contraction was less sensitive to brefeldin A inhibition. Pre-treatment with brefeldin A (30-100 microM) did not affect phenylephrine-induced transient contractions in Ca2+-free medium, but strongly inhibited the phenylephrine-induced sustained contractions upon re-admission of Ca2+ to the medium. Brefeldin A was unable to prevent Ca2+ stores refilling. We concluded that brefeldin A inhibits Ca2+ entry but not the pathways activated after Ca2+ stores depletion or the pathways responsible for replenishment of these stores in rat aorta, presumably by disassembling the Golgi apparatus network.

Functional evidence of inverse agonism in vascular smooth muscle

1. In the present study, depletion of internal Ca2+ stores sensitive to noradrenaline (1 microM) in rat aorta, is the signal for the entry of extracellular Ca2+, not only to refill the stores but also, in our experimental conditions, to activate the contractile proteins. This induces an increase in the resting tone that constitutes, the first functional evidence of this Ca2+ entry. 2. The fact that methoxamine (100 microM) reproduces the same processes as noradrenaline but clonidine (1 microM) does not, indicates that alpha(1)-adrenoceptor activation is related to the increase in the resting tone observed after depletion of adrenoceptor-sensitive internal Ca2+-stores. 3. Benoxathian and WB 4101 (alpha(1A)- and alpha(1D)-adrenoceptor antagonists) selectively inhibit, in a concentration-dependent manner, this mechanical response observed in absence of the agonist, which suggests that these agents can act as inverse agonists and provide a functional model for studying this phenomenon. Since chloroethylclonidine (100 microM) has no effect on this response, the participation of alpha(1B)-adrenoceptors can be ruled out. 4. Contractile responses to noradrenaline (1 microM) in Ca2+-free medium were selectively blocked by chloroethylclonidine. This suggests that the response to noradrenaline in Ca2+-free medium mainly depends on the activation of the alpha(1B)-adrenoceptor subtype.

Dantrolene blocks the tonic contraction and calcium influx evoked by K+ in ileal longitudinal smooth muscle

1. Dantrolene (10(-5)-10(-4) M) reduced the K+ (60 mM) tonic response in accordance with decreased Ca2+ uptake, as determined by La method, more than the phasic in ileal muscle. 2. After saponin-treatment of the fibers, which leaves the Ca2+ storage sites intact, dantrolene had only a slight effect on the caffeine-induced contraction. In Triton-X-100-treated fibers, in which the Ca2+ release sites are destroyed, dantrolene did not affect the Ca(2+)-induced contraction. 3. The results suggest that dantrolene inhibited the high-K(+)-induced tonic response mainly by inhibiting Ca2+ influx via voltage-dependent Ca2+ channels in ileum without affecting contractile elements. In contrast, dantrolene had only a slight effect on the intracellular Ca2+ release from caffeine-sensitive release sites.

Multiple effects of tyrosine kinase inhibitors on vascular smooth muscle contraction

The effects of three tyrosine kinase inhibitors: genistein, quercetin and psi-tectorigenin, were investigated on contractions evoked in de-endothelised rat aortic rings, either by phenylephrine or 70 mM K+. A dose-dependent inhibition of both contractions by all three compounds was observed, the phenylephrine-mediated contractions being more sensitive to genistein. No differences between genistein or quercetin effects in pre-treatment or post-treatment protocols were found. Ca2+ store refilling, expressed in terms of phenylephrine-induced tension in Ca(2+)-free medium, was dose-dependently blocked by quercetin and genistein. Sodium orthovanadate, an inhibitor of tyrosine phosphatase, contracted the rat aortic rings with an IC50 of 0.66 microM. Its presence during the refilling period after exposure to Ca(2+)-free medium completely prevented the subsequent response to phenylephrine. One can conclude that the use of the above-mentioned protein tyrosine kinase inhibitors in the rat aorta blocks a step involved in Ca2+ entry and Ca2+ store refilling. A definite conclusion regarding the vanadate effects is not possible due to the fact that this compound also affects Ca2+ ATP-ases.

Study of the in vivo and in vitro cardiovascular effects of (+)-glaucine and N-carbethoxysecoglaucine in rats

1. The cardiovascular and vasorelaxant effects of (+)-glaucine and of a semisynthetic derivative (N-carbethoxysecoglaucine) were studied in rats. 2. N-carbethoxysecoglaucine did not modify either systolic arterial pressure or heart rate values in conscious (25 mg kg-1, p.o.) and anaesthetized normotensive rats (5 mg kg-1, i.v.). Furthermore, this compound showed no activity in the experiments carried out on rat isolated aorta [contractility and 45Ca2+ influx assays (5 microM)] and did not modify the rate and force of contraction in rat isolated atria (5 microM). 3. In conscious normotensive rats, oral administration of (+)-glaucine (25 mg kg-1) did not modify either systolic arterial pressure or heart rate. 4. In anaesthetized normotensive rats, (+)-glaucine (5 mg kg-1, i.v.) produced a remarkable fall in mean arterial pressure (MAP) accompanied by a significant decrease in heart rate. In the same preparation, (+)-glaucine (5 mg kg-1, i.v.) did not modify the cardiovascular effects induced by noradrenaline (NA) (5 micrograms kg-1) and 5-hydroxytryptamine (5-HT) (300 micrograms kg-1) but markedly inhibited those induced by nicotine (200 micrograms kg-1). 5. In isolated intact aorta of rat, (+)-glaucine (0.15-5 microM) competitively inhibited the contractions induced by NA (with a pA2 value of 7.14) and non-competitively those induced by 5-HT (in normal Krebs solution) and Ca2+ (in depolarizing Ca(2+)-free high-K+ 50 mM solution), with depression of the maximal response and with pD2 values of 5.56 and 5.26, respectively. 6. In experiments in Ca2+-free medium, (+)-glaucine (3 microM) inhibited the contractions induced by NA and had no effect on either 5-HT- or caffeine-induced contractions.7. Furthermore, in the experiments with radioactive Ca2+, (+)-glaucine (3 microM) did not modify the basal uptake of 45Ca2+ but strongly inhibited the influx of 45Ca2+ induced by NA, 5-HT and K+.8. (+)-Glaucine (5microM) had no effect on rate and force of contraction in rat isolated atria.9. These results indicate that: (a) the cardiovascular effects (hypotension and bradycardia) of (+)-glaucine in anaesthetized normotensive rats (5 mg kg-1) may be due, at least in part, to a ganglioplexic effect; (b) the vasorelaxant action of ( + )-glaucine (0.15-5 microM) in rat isolated aorta can be attributed to an alpha1-adrenoceptor blocking property (which may explain its inhibition of noradrenaline-induced 45Ca2+influx and contractions in normal Krebs solution and noradrenaline-induced contractions in Ca2+-free medium) and to a Ca2+-antagonist activity (which may be responsible, at least in part, for the inhibition of 45Ca2+ uptake induced by NA, 5-HT and K+ and the contractions induced by both NA and 5-HT in normal Krebs solution and by Ca2+ in Ca2+-free high-K+ medium) and (c) there is no correlation between the mechanisms of action observed for (+ )-glaucine in vivo and in vitro, which suggests that the vasorelaxant activity of this alkaloid does not contribute to its hypotensive activity.

Zinc ions block the intracellular calcium release induced by caffeine in guinea-pig taenia caeci

Zn2+ in low concentrations (0.005-0.1 mM) inhibited the transient contractions in response to caffeine (25 mM) in a dose-dependent manner in smooth muscle of intact guinea-pig taenia caeci. At Zn2+ concentrations higher than 0.1 mM, caffeine did not elicit any response. After saponin-treatment of the fibres, which leaves the Ca2+ storage sites intact, caffeine contraction was completely inhibited by Zn2+ at a relatively low concentration (0.003 mM). However, in Triton-X-100-treated fibres, in which the Ca2+ release sites are destroyed, the contraction could be induced in the presence of Zn2+ by an increase in Ca2+. In conclusion, Zn2+ can block the intracellular Ca2+ release from caffeine-sensitive release sites in taenia caeci.

Factors controlling changes in intracellular Ca2+ concentration produced by noradrenaline in rat mesenteric artery smooth muscle cells

1. The intracellular Ca2+ concentration ([Ca2+]i) was measured in mesenteric artery smooth muscle cells using the fluorescent indicator indo-1. 2. Noradrenaline (1-10 microM) produced a transient increase in [Ca2+]i. This response was unaffected by the removal of external calcium suggesting that the bulk of the increase in [Ca2+]i produced by noradrenaline is due to release from an intracellular store. 3. The maintained application of caffeine (10 mM) produced a transient rise in [Ca2+]i. The rate of relaxation was slower than that of the noradrenaline response. If caffeine was removed at the peak of the rise in [Ca2+]i then [Ca2+]i recovered more quickly than was the case in both the maintained response to noradrenaline and that to caffeine. 4. In the presence of noradrenaline, caffeine or thapsigargin elevated [Ca2+]i. However, if thapsigargin or caffeine was added first, the subsequent application of noradrenaline did not increase [Ca2+]i, suggesting that only part of the caffeine-sensitive store is sensitive to noradrenaline. 5. The recovery of [Ca2+]i during the application of caffeine was unaffected by the removal of external sodium suggesting that Na+-Ca2+ exchange is not important in the reduction in [Ca2+]i. The addition of lanthanum (1 mM) did, however, greatly slow [Ca2+]i recovery. 6. We conclude that the three major factors responsible for removing Ca2+ ions from the cytoplasm are: (i) a caffeine- and noradrenaline-sensitive store (43%), (ii) a caffeine-sensitive but noradrenaline-insensitive store (36%), and (iii) a sarcolemmal Ca(2+)-ATPase (16%). Finally, a 5% contribution remains to be accounted for.

Mechanism of the cardiovascular activity of laudanosine: comparison with papaverine and other benzylisoquinolines

1. The activity of (+/-)-laudanosine, a benzyltetrahydroisoquinoline alkaloid, was investigated in pithed rats and rat isolated aorta. Its effects on [3H]-(+)-cis-diltiazem and [3H]-nitrendipine binding to rat cerebral cortical membranes, and on the different molecular forms of cyclic nucleotide phosphodiesterases (PDE) isolated from bovine aorta were investigated. 2. The dose-response curve to methoxamine (3-300 micrograms kg-1, i.v.) in normotensive pithed rats was shifted to the right by (+/-)-laudanosine, 3 and 6 mg kg-1. 3. (+/-)-Laudanosine inhibited in a concentration-dependent manner the contractile responses evoked by noradrenaline (NA 1 microM), depolarizing solution (KCl 80 mM) or depolarizing solution plus phentolamine (10 microM) in rat isolated aorta. The alkaloid appeared to be more potent against NA-induced contractions. 4. In Ca(2+)-free solution, (+/-)-laudanosine (100 microM) inhibited the contraction evoked by NA and did not modify the phasic contractile response evoked by caffeine. The alkaloid did not modify the refilling of the intracellular Ca(2+)-sotres sensitive to NA or caffeine. 5. (+/-)-Laudanosine inhibited [3H]-prazosin binding to cortical membranes and also inhibited [3H]-(+)-cis-diltiazem but with a lower potency. [3H]-nitrendipine binding was not affected by laudanosine. 6. (+/-)-Laudanosine does not have a significant effect on the different forms of PDEs isolated from bovine aorta. In contrast, compounds structurally related to this alkaloid such as papaverine and its derivatives, had a non-selective or more specific inhibitory effect on these PDE forms. These differences can be explained on the basis of their structural features: the planarity of the isoquinoline ring(papaverine) facilitates the interaction with receptor sites, and the different position of the benzyl group does not modify the activity unless this position leads to the presence of a chiral centre (laudanosine).7. These results suggest that (+/-)-laudanosine has a selective activity as an alpha1-adrenoceptor blocker. Its lack of action on different PDE forms provides us with information about a group of benzylisoquinolines that with small structural changes show a different effect on PDE-forms isolated from vascular smooth muscle.

Inhibitory effects of quercetin and staurosporine on phasic contractions in rat vascular smooth muscle

The aim of this work was to analyze the effects of quercetin and staurosporine on the phasic contractile responses in rat aorta induced by noradrenaline, 5-hydroxytryptamine (5-HT, serotonin) and caffeine in Ca(2+)-free media. Both quercetin and staurosporine inhibited the contractions induced by 10(-5) M noradrenaline, 10(-5) M 5-HT and 20 mM caffeine in Ca(2+)-free solution. Phorbol 12-myristate 13-acetate (5 x 10(-8) M) enhanced this transient contraction elicited by noradrenaline, an effect that was abolished by quercetin (5 x 10(-5) M). The relaxant effects of quercetin on 80 mM KCl induced contractions were similar in normal and low Na+ solution, e.g. when Ca2+ efflux through the Na+/Ca2+ exchanger was inhibited. Furthermore, quercetin or staurosporine had no effect on 45Ca2+ efflux under resting conditions or when stimulated by 10(-5) M noradrenaline. These results suggested that the inhibitory effects of quercetin and staurosporine on phasic contractile responses induced by receptor agonists in Ca(2+)-free media do not seem to be related to changes in cellular Ca2+ regulation but to an inhibitory effect on the regulation of contractile proteins, an effect probably related to the decreased sensitivity of contractile elements to Ca2+ that apparently resulted from the inhibitory effects of quercetin and staurosporine on protein kinases.

Caffeine and exercise: metabolism and performance

Caffeine ingestion prior to prolonged exercise delays fatigue. However, the mechanisms involved are very unclear. Caffeine is associated with elevated plasma epinephrine but the metabolic impact of this is uncertain. Glycogen sparing occurs in active muscle, at least in the first few minutes, but studies have generally failed to demonstrate enhanced fat metabolism. The demethylation of caffeine by the hepatic cytochrome P-450 oxygenases begins within minutes and dimethylxanthines (especially paraxanthine) are generated. These compounds appear in the plasma within an hour of caffeine ingestion and may have effects on tissues that have been attributed to caffeine and/or epinephrine. While the most widely supported theory is that caffeine and other methylxanthines are adenosine receptor antagonists, this action alone cannot explain all of the observed responses. Nevertheless, habituation to and withdrawal from caffeine are associated with up and down regulation of adenosine receptors. One study demonstrated marked differences in the effects of caffeine on the plasma concentrations of epinephrine and dimethylxanthines between caffeine users and nonusers. Caffeine is clearly a very active drug that has many effects on humans including increasing exercise endurance. This can be associated with muscle glycogen sparing and elevated plasma epinephrine, but the underlying mechanisms are unknown.

Effects of okadaic acid on the human isolated bronchus

The effects of okadaic acid, a polyether derivative of a 38-carbon monocarboxylic fatty acid obtained from a culture of the marine dinoflagellate, Prorocentrum lima, were studied on the human isolated bronchus. In low concentrations (0.01 and 0.03 microM), okadaic acid had no significant effect of its own on the human isolated bronchus, but in higher concentrations (0.1-10 microM) it induced a series of contractions and relaxations. The first contraction was of low intensity (5% of maximum response to acetylcholine 3 mM) and occurred early. The second contraction had a higher amplitude (30% of maximum response to acetylcholine 3 mM) and reached its peak with okadaic acid 0.3 microM. At higher concentrations (1-10 microM), following a relaxation phase, a later rebound contraction occurred between 70 and 120 min and corresponded to 40% of the maximum response to acetylcholine 3 mM. In addition, okadaic acid inhibited or abolished the contractile response evoked by either KCl 60 mM or acetylcholine 3 mM with IC50 of 0.04 and 0.12 microM, respectively. The second contraction evoked by 0.3 microM okadaic acid was partially inhibited in the presence of the Ca2+ channel blocker, nicardipine 1 microM, or after incubation of the human bronchus in a Ca(2+)-free solution and it was completely abolished in the presence of CdSO4 0.1 mM.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of endothelium-derived relaxing factor involvement in the potentiating effect of parathyroidectomy on norepinephrine-induced rat aortic contraction

Previous results have shown that the contractile response to norepinephrine (NE) was enhanced in isolated aortae from SHR and normotensive Wistar parathyroidectomized rats. In this work we sought to characterize the contribution of endothelium-derived relaxing factor (EDRF) release to this effect which is not linked to hypertension. Parathyroidectomy (PTX) was performed by surgery on 5 week-old male Wistar rats. Five weeks later intact (E+) and rubbed (E-) aortic rings were mounted in an organ chamber for isometric tension recording. KCl-induced contractions were potentiated in PTX E+ aortae compared to sham operated (SO), (P < 0.05), but not in denuded E- aortae. Similarly NE (1 nM- 10 microM) induced a potentiated contractile response in PTX E+ (P < 0.01), but not in PTX E- rings; nevertheless the sensitivity did not change. After removal of endothelium, the expected enhanced contraction and sensitivity observed in SO rats was not present in PTX. The NO synthase inhibitor L-NAME (20 microM), enhanced sensitivity to NE in SO but not in PTX E+ aortic rings. In addition, hemoglobin (Hb, 10 microM) enhanced NE contraction in SO (P < 0.01) aortic rings, but to a lesser extent in PTX rat aortae. Moreover, in the presence of L-NAME or Hb, SO and PTX aortae displayed a similar contraction. Superoxide dismutase (SOD, 150 U/ml) diminished the NE contraction since NO was protected from degradation but the difference was still present between SO and PTX rat aortae, ruling out the possible implication of superoxide anions in the hyperreactivity of PTX aortae. On the other hand, A23187, which induces EDRF release, reduced the level of NE contraction as expected, but suppressed the PTX enhancing effect and in calcium-free solution the enhancement of contraction after PTX was not observed. These experiments extend to the rat the observations previously obtained in rabbit aorta: extracellular calcium is a major determining factor in NO production. Acetylcholine and A23187 (cumulative doses) produced an endothelium-dependent relaxation which was not significantly modified in NE-pre-contracted PTX aortae compared to SO aortae. L-arginine (100 microM), reversed the L-NAME inhibitory effect and induced an attenuated endothelium-dependent relaxation in PTX vessels (P < 0.01). In conclusion, in rat isolated aortae the enhancing effect of parathyroidectomy on norepinephrine and KCl contractions is due to a diminished endothelial nitric oxide production. This might arise via a decrease of the constitutive NO synthase activity in an extracellular calcium-dependent manner.

The effect of EDTA on contractile responses of guinea-pig trachea in calcium-free medium and on the recovery of the contractile responses in calcium-containing solution

1. Acetylcholine (0.1 mmol/l) and KCl (80 mmol/l) induce contractile responses in guinea-pig trachea. 2. In the absence of extracellular Ca, contractile responses to ACh or KCl decrease as the preincubation time or the EDTA concentration increases. 3. Exposure of the tissue to Ca-free medium decreases the basal tone. 4. Prolonged incubation in the presence of EDTA (1 mmol/l) promotes a delay in the recovery of the tonic component of the contraction elicited subsequently by acetylcholine and KCl in physiological salt solution, and abolishes the phasic one.

Evidence that depletion of internal calcium stores sensitive to noradrenaline elicits a contractile response dependent on extracellular calcium in rat aorta

1. Noradrenaline 1 microM induced a contractile response in rat isolated aorta in the presence or in the absence of extracellular Ca2+ with depletion of intracellular Ca2+ stores. Thereafter, during incubation in the presence of Ca2+, an increase in the resting tone was observed. Such a contractile response did not occur after exposure to caffeine or 5-hydroxytryptamine. 2. This increase in tension was inhibited in a concentration-dependent manner by alpha-adrenoceptor antagonists (prazosin, phentolamine and yohimbine), the non-specific relaxing compound, papaverine and by the Ca(2+)-entry blocker, nifedipine. Therefore, this contractile process is related to depletion of Ca2+ stores sensitive to noradrenaline and is linked to Ca2+ entry through voltage-operated Ca2+ channels and alpha-adrenoceptors. 3. Phentolamine and yohimbine did not block the Ca2+ refill pathway; prazosin and nifedipine inhibited the reuptake of Ca2+ by an internal store sensitive only to noradrenaline; papaverine inhibited the refilling of caffeine- and noradrenaline-sensitive Ca(2+)-stores.

Effects of different agents on the contractile response elicited by extracellular calcium after depletion of internal calcium stores in rat isolated aorta

Noradrenaline, 1 microM, induced a sustained contractile response in rat isolated aorta in the presence and in the absence of extracellular Ca2+. After depleting the noradrenaline-sensitive intracellular Ca2+ stores, an increase in the basal tone of the aorta was observed during the incubation period in the presence of Ca2+ and in the absence of the agonist. We have tested the possible pathways through which Ca2+ enters the cell to refill the previously depleted Ca2+ pools, a process that is accompanied by an increase in tension. The magnitude of this increase does not depend on the presence of Mg2+ in the extracellular medium nor on the temperature, suggesting that it is mediated by an event that does not depend on intracellular energy or Ca2+, Mg(2+)-ATPase. It is inhibited in a concentration-dependent manner by an unspecific relaxing compound, caffeine, and an organic Ca2+ entry blocker, verapamil, but not by an inorganic Ca2+ entry blocker, lanthanum. Caffeine (10 mM) and verapamil (10(-5) M) completely inhibited the increase in the resting tone, but only verapamil abolished the refilling of the noradrenaline-sensitive Ca2+ pools, indicating that the extracellular Ca2+ enters the cell through voltage-operated Ca2+ channels. Caffeine inhibited the increase in the resting tone without blocking the refilling process of the stores at 37 degrees C, but at 25 degrees C a partial inhibition of the repletion of internal Ca2+ pools was observed. These results confirm previous work that showed a temperature-dependent activity of caffeine.

Modulatory role of magnesium on the contractile response of rat aorta to several agonists in normal and calcium-free medium

Acute withdrawal of external Mg2+ increased basal tone of rat isolated aorta incubated in the presence of Ca2+. Above normal levels of Mg2+ (1-4 mM) inhibited basal tone while much higher levels of the divalent cation (64-256 nM) evoked contractile responses regardless of the presence of Ca2+. Contractile responses to noradrenaline (1 microM) and KCl (80 mM) were inhibited by addition of cumulative concentrations of Mg2+. Acetylcholine-induced contractions in the presence of physiological concentrations of Mg2+ (1 mM) decreased gradually to the basal tone, but a sustained contraction was observed in the absence of this ion. In Ca(2+)-free medium, acetylcholine-induced phasic responses indicate the existence of an acetylcholine-sensitive Ca2+ store. KCl induced contraction only in Krebs solution, although a small residual contraction could be observed in Ca(2+)-free medium in some experiments. Mg(2+)-depletion in the extracellular medium increased contractile responses induced by acetylcholine and KCl in Ca(2+)-free medium. These results suggest that extracellular Mg2+ modulates basal tone, Ca2+ channels and responsiveness to various agents in the absence of Ca2+.

Investigations of the dual contractile/relaxant properties showed by antioquine in rat aorta

1. In the present study we assessed the activity of antioquine, a bisbenzyltetrahydroisoquinoline alkaloid isolated from Pseudoxandra sclerocarpa, by examining its effects on the contractile activity of rat isolated aorta, specific binding of [3H]-(+)-cis-diltiazem, [3H]-nitrendipine and [3H]-prazosin to cerebral cortical membranes and the different molecular forms of cyclic nucleotide phosphodiesterases (PDE) isolated from bovine aorta. 2. Contractions in rat aorta induced by high concentrations of KCl (80 mM) and noradrenaline (1 microM) were inhibited by antioquine in a concentration-dependent manner (0.1 microM- 300 microM). The alkaloid appeared more potent against KCl-induced contractions. This inhibitory effect was observed at both 37 degrees C and 25 degrees C. 3. Paradoxically, at the highest concentration tested (300 microM) antioquine induced a contractile response of similar magnitude in the presence and absence of extracellular calcium, at 37 degrees C. This activity was greatly attenuated at 25 degrees C. Antioquine-induced contractions were not inhibited by prazosin (0.1 microM), nifedipine (1 microM) or diltiazem (100 microM). On the contrary, prazosin and nifedipine slightly increased the contractions in the presence of extracellular calcium. Papaverine (100 microM) partially inhibited the contractile response to antioquine both in the presence and absence of extracellular calcium. 4. At 25 degrees C, in Ca(2+)-free solution, antioquine (300 microM) did not modify the contractile response (phasic and tonic) evoked by noradrenaline, but increased the phasic contraction induced by caffeine. At 37 degrees C, the contraction elicited by antioquine made it impossible to observe the noradrenaline-induced one. 5. Antioquine showed affinity for the [3H]-prazosin binding site and for the [3H]-(+)-cis-diltiazembinding site of the Ca2+-channel receptor complex, but had no effect at the dihydropyridine binding site in rat cerebral cortex.6. Antioquine weakly inhibited some PDE forms isolated from bovine aorta: a CaM-PDE (PDE I)which preferentially hydrolyzes cyclic GMP and is activated by calmodulin, and a rolipram-sensitive cyclic AMP-PDE (PDE IV) which hydrolyzed cyclic AMP. Antioquine did not exert any inhibitory effect on the other forms of PDE, a cyclic GMP selective form (PDE V) and a low Km cyclic AMP-PDEthat is inhibited by cyclic GMP (CGI-PDE, PDE III).7. The present work provides evidence that antioquine has properties both as a calcium entry blocker(possibly through the benzothiazepine recognition site in the calcium channel) and as a contractile agent.Its mechanism of action as a contractile agent is not related to Ca2+-entry and is hypothetically similar to that of calyculin-A or okadaic acid. The possible involvement of a-adrenoceptors in this paradoxical effect cannot be excluded. The rigidity of the molecule provides an interesting model for analyzing this contractile mechanism and the intracellular processes involved.

Amplifying effect of serotonin on contractile responses in rat aorta and depletion of intracellular Ca-stores

1. Serotonin, 1 microM, induces a contractile response in isolated rat aorta in the presence or absence of extracellular Ca. 2. In Ca-free media, the fast phasic contraction is lower in magnitude and further addition of serotonin evokes no response. 3. Recovery of the contractile response in Ca-free medium is obtained by a 40 min incubation in Ca-containing solution. 4. In Ca, Mg-free medium, the response to serotonin is significantly higher than that obtained in the presence of Mg. 5. An amplifying effect of serotonin on the contractile responses induced by serotonin itself or by noradrenaline was observed in Ca-containing but not in Ca-free solution.

Calcium ion homeostasis in smooth muscle

Ca2+ plays an important role in the regulation of smooth-muscle contraction. In this review, we will focus on the various Ca(2+)-transport processes that contribute to the cytosolic Ca2+ concentration. Mainly the functional aspects will be covered. The smooth-muscle inositol 1,4,5-trisphosphate receptor and ryanodine receptor will be extensively discussed. Smooth-muscle contraction also depends on extracellular Ca2+ and both voltage- and Ca(2+)-release-activated plasma-membrane Ca2+ channels will be reviewed. We will finally discuss some functional properties of the Ca2+ pumps that remove Ca2+ from the cytoplasm and of the Ca2+ regulation of the nucleus.

Inhibitory and contractile effects of okadaic acid on rat uterine muscle

The effects of okadaic acid and its interactions with various agents known to increase, by different mechanisms, the intracellular levels of cyclic AMP and/or cyclic GMP were investigated in isolated strips of rat myometrium. Okadaic acid showed inhibitory effects at concentrations between 10(-7) M and 3 x 10(-6) M. At higher concentrations, a biphasic, contractile and then relaxant response was observed. The results obtained suggest that, in rat uterine smooth muscle, the inhibitory effects of okadaic acid are not entirely mediated by the activation of cyclic AMP- and/or cyclic GMP-dependent pathways. The data also point to the existence of a clear interaction between okadaic acid and methylxanthines, although further studies are needed to clarify the mechanisms involved in this interaction.

Multiple actions of glaucine on cyclic nucleotide phosphodiesterases, alpha 1-adrenoceptor and benzothiazepine binding site at the calcium channel

1. In the present study, the properties of glaucine (an aporphine structurally related to papaverine) were compared with those of papaverine, diltiazem, nifedipine and prazosin. The work includes functional studies on rat isolated aorta contracted with noradrenaline, caffeine or KCl, and a determination of the affinity of glaucine at calcium channel binding sites of alpha-adrenoceptors, by use of [3H]-(+)-cis-diltiazem, [3H]-nitrendipine and [3H]-prazosin binding to cerebral cortical membranes. The effects of glaucine on the different molecular forms of cyclic nucleotide phosphodiesterases (PDE) isolated from bovine aorta were also determined. 2. Contraction evoked by noradrenaline (1 microM) or depolarizing solution (60 mM KCl) were inhibited in a concentration-dependent manner by all the compounds tested. As expected, prazosin showed a greater selectivity of action on NA-induced contraction, whereas nifedipine and diltiazem appeared more potent on KCl-induced contraction. Glaucine had a greater potency on the contraction elicited by noradrenaline whereas papaverine acted non specifically. 3. In Ca(2+)-free solution, prazosin (0.1 microM) and glaucine (0.1 mM) inhibited the contraction evoked by NA; diltiazem (0.1 mM) diminished this contraction whereas nifedipine (1 microM) had no effect. Preincubation of tissues with glaucine, diltiazem, nifedipine and prazosin did not modify the contractile response induced by caffeine. In contrast, papaverine (0.1 mM) significantly inhibited the contractions evoked by NA or caffeine in Ca(2+)-free medium. 4. Glaucine and papaverine show affinity at the [3H]-prazosin binding site and at the benzothiazepine binding site of the Ca(2+)-channel receptor complex, but have no effect at the dihydropyridine binding site in rat cerebral cortex. Glaucine exerts some selectivity as an inhibitor of [3H]-prazosin binding as opposed to [3H]-(+ )-cis-diltiazem binding while papaverine appears to have approximately equal affinity in this respect.5. This study confirms the presence of four phosphodiesterase (PDE) activities in bovine aorta: a calmodulin-activated PDE (CaM-PDE type I) which hydrolyzed preferentially guanosine 3':5'-cyclic monophosphate (cyclic GMP); a cyclic GMP selective form (cGMP-PDE type V); and two low Km adenosine 3':5'-cyclic monophosphate (cyclic AMP) PDEs that are insensitive to the stimulatory effect of CaM, one of which was inhibited by cyclic GMP (CGI-PDE, type III) and the other by rolipram (cAMP-PDE, type IV). Glaucine selectively inhibits one of the two forms of Ca2+-independent low Km cAMP-PDE, the type IV. In contrast, papaverine exerts a non-selective inhibitory effect upon all PDE forms.6. The present work provides evidence that glaucine, a benzyltetrahydroisoquinoline alkaloid, has interesting properties as an alpha l-adrenoceptor antagonist, calcium entry blocker (through the benzothiazepine recognition site in the calcium channel) and as a selective inhibitor of the rolipram-sensitive cAMP-PDE, type IV PDE.