Mjmc Thoolen

Effect of converting enzyme inhibition and angiotensin receptor blockade on the vasoconstriction mediated by alpha 1-and alpha 2-adrenoceptor stimulation in pithed normotensive rats

SummaryThe effect of functional impairment of the renin-angiotensin system on the vasoconstriction mediated by postsynaptic α1 and α2-adrenoceptors in pithed normotensive rats was studied. Selective α1-adrenoceptor stimulation was induced by intravenously administered cirazoline, whereas B-HT 920 was used as a selective agonists at α2-adrenoceptors. The angiotensin converting enzyme was inhibited by intravenous treatment of the pithed rats with captopril, teprotide or enalapril. Blockade of angiotensin receptors was produced by intravenously applied [Sar1 Ala8]angiotensin II (saralasin). Pretreatment with angiotensin converting enzyme inhibitors or with saralasin in doses which produced a maximal reduction in basal diastolic blood pressure, only slightly attenuated the hypertensive response to cirazoline. In contrast, these drugs provoked a most significant reduction of the α2-adrenoceptor mediated vasoconstriction. Restoration of the basal diastolic blood pressure by intravenous infusion with angiotensin II or with vasopressin completely reversed the inhibitory effect of captopril on the vasopressor response to B-HT 920. One hour after bilateral nephrectomy, captopril still reduced the α2-adrenoceptor mediated vasoconstriction. However, 18–24 h after bilateral nephrectomy, captopril had no additional inhibitory effect on the vasopressor response to selective α2-adrenoceptor stimulation. It is concluded that in pithed normotensive rats the pressor response to α2-adrenoceptor stimulation is significantly potentiated by endogenous angiotensin II, even at low circulating levels of the octapeptide. The modulatory action of angiotensin II on the α-adrenoceptor mediated vasoconstriction probably represents an effect on the basal arteriolar muscular tone rather than a specific interaction.

DIFFERENTIAL EFFECT OF CALCIUM ENTRY BLOCKERS ON ALPHA-1-ADRENOCEPTOR-MEDIATED VASOCONSTRICTION INVIVO

SummaryThe effects of the calcium entry blockers nifedipine, (−)-verapamil and the dihydropyridine derivative PY 108-068 were evaluated on the increase in diastolic pressure of pithed normotensive rats caused by the selective α1-adrenoceptor agonists cirazoline, (−)-phenylephrine, (±)-erythro-methoxamine, (−)-amidephrine and St 587 [(2-chloro-5-trifluoromethylphenylimino)-2-imidazolidine] as well as by the mixed α1/α2-adrenoceptor agonists clonidine and DPI [(3,4-dihydroxyphenylimino)-2-imidazolidine]. The calcium entry inhibitors (up to 3 mg/kg) caused 3- to 5-fold, parallel rightward shifts of the log dose-pressor effect curves to cirazoline, (−)-phenylephrine, (±)-erythro-methoxamine and (−)-amidephrine accompanied by only a slight depression of the maximal pressor response. In contrast, the calcium entry inhibitors produced a dose-dependent profound depression of both maximum and slope of the log dose-pressor response curves to St 587 and clonidine. For DPI about 10-and 100-fold parallel displacements to the right without reduction of the maximum were found following treatment with 1 and 3 mg/kg of nifedipine, respectively. Infusion of vasopressin to counteract the vasodilatory action produced by the calcium entry inhibitors did not significantly change the pattern of interference observed under the conditions of decreased baseline diastolic pressure. The results indicate that α1-adrenoceptor-mediated vasoconstriction in the pithed normotensive rat, which is characterized by its sensitivity to blockade by prazosin and its relative insensitivity to antagonism by yohimbine or rauwolscine, can be subdivided into two distinct processes which are differentially influenced by blockade of calcium entry. It is suggested that cirazoline, (−)-phenylephrine, (±)-erythro-methoxamine and (−)-amidephrine initiate vasoconstriction predominantly by a mechanism independent of extracellular calcium influx. On the other hand, St 587 and clonidine increase arterial pressure via a mechanism which seems to be mainly governed by an entry of extracellular calcium ions. DPI probably triggers both processes of vasoconstriction in the intact circulatory system of the pithed normotensive rat.

Captopril impairs the vascular smooth muscle contraction mediated by postsynaptic α2-adrenoceptors in the pithed rat

Since two different types of α-adrenoceptors (α1- and α2-) subserve vasoconstriction in pithed rats and noradrenaline behaves as a non-selective agonist of both receptors (for review see Timmerman and van Zwieten, 1981), we have investigated this interaction between noradrenaline and captopril in more detail to identify the class of α-adrenoceptor involved. This study confirms the findings reported recently (Antonaccio and Kerwin, 1981; Clough et al., 1981) that captopril inhibits postjunctional vasocontrictor responses to noradrenaline in pithed rats. However, captopril interfered with the vasoconstriction elicited by noradrenaline only in those animals in which the vascular postsynaptic α2-adrenoceptor system could be triggered (saline-and prazosin-treated rats). Captopril was without effect after removal of the contribution of post-synaptic α2-adrenoceptor stimulation to the in vivo vascular smooth muscle contraction due to (-)-noradrenaline (rauwolscine-treated rats). These results indicate that captopril selectively impairs the vasoconstriction subserved in vivo by postjunctinal α2-adrenoceptors. The vasoconstrictor process initiated by α1-adrenoceptor activation is not affected by captopril.

EFFECT OF CAPTOPRIL ON SYMPATHETIC NEUROTRANSMISSION IN PITHED NORMOTENSIVE RATS

Captopril significantly diminished the basal diastolic blood pressure and the vasopressor response to electrical stimulation of the thoracic-lumbar spinal cord in pithed normotensive rats. The reduction of the hypertensive response to electrical stimulation was more pronounced after bilateral adrenalectomy. Captopril also diminished the vasopressor response to intravenously administered (-)-noradrenaline. Pretreatment of the animals with indomethacin had no effect on the vasopressor response to electrical stimulation and did not affect the sympathoinhibition of captopril. After bilateral nephrectomy, 18-24 h previously, the basal diastolic blood pressure and the vasopressor response to electrical stimulation were reduced and captopril had no additional inhibitory effect on these parameters. In indomethacin-pretreated animals with intact kidneys, restoration of the basal diastolic blood pressure with angiotensin II (AII) completely abolished the sympathodepressive effect of captopril. When the reduction in basal diastolic blood pressure with captopril was prevented by vasopressin, converting enzyme inhibition had no depressive effect on the hypertensive response to intravenously administered (-)-noradrenaline and did not influence sympathetic neurotransmission in animals with intact adrenals. However, a small, but significant reduction of the hypertensive response to electrical stimulation by captopril was still detectable in bilaterally adrenalectomized rats. The results suggest that endogenous AII facilitates sympathetic neurotransmission in vascular smooth muscle of pithed normotensive rats. However, the modulatory action of endogenous AII largely results from an effect on basal arteriolar smooth muscle tone and should, therefore, be considered as non-specific facilitation. Genuine prejunctional facilitation of the sympathetic neurotransmission in vascular smooth muscle can only be observed after bilateral adrenalectomy but this effect of endogenous AII appears of minor significance, at least in pithed normotensive rats.

Hypotensive Activity of Tetrandrine in Rats

The influence of tetrandrine on arterial pressure and heart rate of pentobarbitone-anaesthetized and conscious normotensive and hypertensive rats (SH rats, renal hypertensive rats, DOCA-salt hypertensive rats) was investigated. Tetrandrine administered intravenously (i.v.) to these animals caused an acute, long-lasting and dose-dependent decrease in mean arterial pressure. Heart rate was not significantly altered. In pithed rats, tetrandrine injected intraarterially 15 min previously impaired the increase in diastolic pressure induced by i.v. B-HT 920, a highly selective α2-adrenoceptor-stimulating agent, in a dose-dependent manner. In a low dose, a parallel displacement to the right was observed, whereas for higher doses the shift was non-parallel. A high dose of tetrandrine shifted the pressor response curve of the αpadrenoceptor agonist, methoxamine, only slightly to the right and did not depress the maximal response. Tetrandrine also caused a moderate rightward shift of the vasopressor effect curve of vasopressin applied i.v. Tetrandrine displayed only minor affinities for specific binding sites in rat brain membranes for [3H]-prazosin (α1-adrenoceptors) and for [3H]-clonidine (α1-adrenoceptors). The results obtained suggest that the hypotensive effect of tetrandrine may be related to an impairment of vasoconstrictor tone mediated by vascular postsynaptic α2-adrenoceptors.

Effects of the irreversible α-adrenoceptor antagonists phenoxybenzamine and benextramine on the effectiveness of nifedipine in inhibiting α1- and α2-adrenoceptor mediated vasoconstriction in pithed rats

SummaryIn pithed normotensive rats, i.v. injection of the selective α1-adrenoceptor agonist cirazolien produced vasoconstriction which was largely resistant to inhibition by nifedipine. On the other hand, the pressor effects of the selective α1-adrenoceptor agonists St 587 and Sgd 101/75 were much more effectively blocked by nifedipine, although not as effectively as the pressor effects to the selective α2-adrenoceptor agonist B-HT 920. The sensitivity to inhibition of vasoconstriction in pithed rats to the different agonists increased in the order cirazoline ≪ St 587<Sgd 101/75<B-HT 920. Phenoxybenzamine (3–300 μg/kg, i.v., −60 min) irreversibly antagonized the vasoconstriction to cirazoline, St 587, Sgd 101/75 and B-HT 920. After treatment of the rats with phenoxybenzamine the potency and efficacy of nifedipine in antagonizing vasoconstriction to α1-, but not to α2-adrenoceptor activation was dose-dependently enhanced. The potency of nifedipine to inhibit α1-vasoconstriction by cirazoline, St 587 and Sgd 101/75 was increased maximally to the level of efficacy at which nifedipine antagonized B-HT 920-induced vasoconstriction. The dose of phenoxybenzamine required to maximally increase the potency and efficacy of nifedipine to antagonize vasoconstriction of the α1-adrenoceptor agonists was inversely related to the level of sensitivity to blockade by nifedipine of the vasoconstriction they produced. In contrast, pretreatment of rats with the irreversible antagonist, benextramine (10 mg/kg, i.v., −100 to −60 min) did not increase the potency or efficacy of nifedipine to antagonize vasoconstriction to cirazoline, St 587, Sgd 101/75 or B-HT 920, despite irreversible blockade of α1- and α2-adrenoceptors. These data suggest that phenoxybenzamine, but not benextramine, selectively inhibits the α1-adrenoceptor mediated vasoconstrictor mechanism that is independent of influx of extracellular calcium. Moreover, the results show that the existence of receptor reserve or the number of α1-adrenoceptors activated does not determine the relative contribution of calcium influx-independent mechanisms in α1-adrenoceptor-mediated vasoconstriction.

Subclassification of muscarinic receptors in the heart, urinary bladder and sympathetic ganglia in the pithed rat

SummaryIn pithed normotensive rats muscarinic receptors were characterized heart, urinary bladder and sympathetic ganglia; the selectivity of some classical muscarinic agents for these subtypes was investigated. The potencies in decreasing heart rate, increasing bladder pressure and increasing diastolic blood pressure were measured for the following, intraarterially administered cholinergic agonists: McN-A-343 ([4-m-chlorophenylcarbamoyloxy]-2-butynyltrimethylammonium), pilocarpine, carbachol, oxotremorine, arecoline, acetyl-β-methylcholine and acetylcholine. The selective M1-antagonist pirenzepine, the mixed M1/M2-antagonist dexetimide and the cardioselective M2-antagonist gallamine were used as tools for identification of the receptors. All data were obtained after intravenous pretreatment with a high dose of atenolol to eliminate tachycardia induced by stimulating sympathetic ganglionic muscarinic receptors.Dexctimide strongly antagonized the bradycardia as well as the increase in bladder pressure induced by pilocarpine, carbachol, oxotremorine, arecoline, acetyl-β-methylcholine and acetylcholine, whereas pirenzepine was much less effective. Gallamine antagonized the bradycardia, whereas no influence was found on the bladder contraction. Pilocarpine acted as a partial agonist in reducing heart rate as well as in increasing bladder pressure, whereas McN-A-343 was almost ineffective in doses up to 1 mg/kg.The hypertensive response to pilocarpine and carbachol was less pronounced than that produced by McN-A-343. Pirenzepine and dexetimide significantly antagonized the hypertensive response to McN-A-343 and pilocarpine, whereas gallamine was much less effective. The hypertensive response induced by carbachol was totally blocked by hexamethonium. The other agonists used in this study did not produce a significant increase in diastolic blood pressure in doses that produced a maximal effect on heart rate and urinary bladder pressure.Simultaneously, intraarterially infused acetylcholine dose-dependently and reversibly decreased the pressor response to intravenously administered McN-A-343.These data suggest that muscarinic receptors in rat sympathetic ganglia belong to the M1-subtype, whereas the muscarinic receptors in rat heart and urinary bladder represent heterogenous populations of M2-receptors. The agonists used in this study, though, could not discriminate between these heterogenous M2-receptors.Like McN-A-343, pilocarpine appears to be a rather selective M1-agonist. In this study the M1/M2 selectivity of muscarinic agents with pronounced M2-agonist activity could not be evaluated since M2-receptor stimulation interferes with the hypertensive response to M1-receptor stimulation.

Inhibitory dopamine receptors on sympathetic neurons innervating the cardiovascular system of the pithed rat

SummaryAdditional experimental evidence was obtained for an inhibitory function of prejunctional α2-adrenoceptors and/or dopamine receptors located on noradrenergic neurons innervating the heart and resistance vessels of the pithed normotensive rat. Mixed α2-adrenoceptor receptor agonists, differing in selectivity towards either receptor type, i.e. N,N-di-n-propyldopamine (DPDA), 2-N, N-di-n-propylamino-6, 7-dihydroxy-1,2,3,4-tetrahydronaphthalene (DP-6,7-ADTN), B-HT 920 and B-HT 933 (azepexole) were used.In pithed normotensive rats, DPDA (30 and 100 μg/kg/min) dose-dependently inhibited the electrical stimulation-induced increase in diastolic pressure, but did not significantly affect the stimulation-evoked increase in heart rate. The inhibition exerted by DPDA was blocked by haloperidol and sulpiride (0.3 mg/kg of each), but not by yohimbine (1 mg/kg), indicating the involvement of dopamine receptors. In this respect, sulpiride and haloperidol were found approximately equipotent.DP-6,7-ADTN (10 and 30 μg/kg/min) impaired both tachycardic and vasoconstrictor responses in a dose-dependent manner. Sulpiride (0.3 mg/kg) only partially restored the DP-6,7-ADTN-depressed stimulation-evoked increase in diastolic pressure, whereas yohimbine (1 mg/kg) alone was without effect. The combination of both antagonists completely prevented the inhibition caused by DP-6,7-ADTN. On the other hand, yohimbine (1 mg/kg), but not sulpiride (0.3 mg/kg), selectively antagonized the DP-6,7-ADTN-induced inhibition of stimulation-evoked tachycardia.B-HT 920 (1, 3 and 10 μg/kg/min) very effectively reduced the increase in diastolic pressure and heart rate caused by electrical stimulation. Inhibitory dopamine as well as α2-adrenoceptors participated in the vascular effects of B-HT 920, whereas α2-adrenoceptors were only involved in the cardioinhibitory response to this agonist.B-HT 933 (0.6 and 1 mg/kg/min) dose-dependently reduced the stimulation-evoked increase in arterial pressure through selective stimulation of inhibitory α2-adrenoceptors, dopamine receptors not taking a part.The results confirm and extend the observations that in addition to α2-adrenoceptors inhibitory dopamine receptors are located on the sympathetic neurons connected with the arterial vasculature of the pithed normotensive rat. The sympathetic nerves innervating the rat heart do not contain inhibitory dopamine receptors; their activity only can be modulated by α2-adrenoceptor stimulation. In the pithed normotensive rat, activation of prejunctionally located α2-adrenoceptors more effectively inhibits the sympathetic activity directed to the heart than that to the resistance vessels.

Invariable susceptibility to blockade by nifedipine of vasoconstriction to various alpha 2-adrenoceptor agonists in pithed rats.

The sensitivity of the increase in diastolic pressure brought about by the selective agonists of α2‐adrenoceptors, B‐HT 920, B‐HT 933, xylazine, UK‐14,304, M‐7, TL‐99 and DP‐6, 7‐ADTN in pithed normotensive rats to blockade by the calcium entry inhibitor nifedipine has been investigated. To exclude any participation of vascular α1‐ and β2‐adrenoceptors, as well as cardiac β1‐adrenoceptors, in the pressor responses, the study was made after treatment of the pithed rats with prazosin (0·1 mg kg−1) and (‐)‐propranol (1 mg kg−1). Without exception, the preferential agonists of α2‐adrenoceptors elicited vasoconstrictor responses which were susceptible to inhibition by nifedipine (0·03–1 mg kg−1) in a dose‐dependent manner regardless of the differences in intrinsic activity of the compounds. The pressor activity was almost completely abolished after 1 mg kg−1 of nifedipine. The results show that vasoconstriction induced in pithed rats by various selective stimulating agents of postjunctional vascular α2‐adrenoceptors is invariably and equally sensitive to attenuation by nifedipine. This susceptibility of α2‐adrenoceptor‐mediated vasoconstriction to impairment by blockade of calcium entry is not dependent on the nature, the potency or the efficacy of the agonist.

Characterization of flufylline, fluprofylline, ritanserin, butanserin and R 56413 with respect to in-vivo α1-, α2- and 5-HT2-receptor antagonism and in-vitro affinity for α1-, α2- and 5-HT2-receptors: comparison with ketanserin

The experimental drugs butanserin (R 53393), ritanserin (R 55667), R 56413, flufylline (Sgd 195/78) and fluprofylline (Sgd 144/80) were evaluated with respect to their antagonism at postjunctional α1‐ and α2‐adrenoceptors and 5‐HT2‐receptors in pithed rats. Moreoever, affinity for [3H]mianserin, [3H]prazosin and [3H]yohimbine binding sites was assessed using rat brain preparations. In all experiments ketanserin was taken as a reference compound. It is concluded that of the compounds investigated butanserin is the most potent and selective α1‐adrenoceptor antagonist, whereas ritanserin was found to be a potent and selective 5‐HT2‐antagonist. Of the other compounds, fluprofylline was a very selective though not very potent α1‐adrenoceptor antagonist. The other compounds were less active and less selective in this respect.