J. C. A. van Meel

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.

The effect of selective α1- and α2-adrenoceptor stimulation on intraocular pressure in the conscious rabbit

SummaryThe influence of topically applied selective α1- and α2-adrenoceptor agonists on intraocular pressure and the diameter of the pupil was investigated in conscious rabbits. Selective stimulation of the α1-subtype of receptors induced an elevation in intraocular pressure, accompanied by mydriasis, whereas stimulation of the α2-subtype caused a marked and dose-dependent ocular hypotensive response, which was blocked by the selective α2-adrenoceptor antagonist yohimbine. α2-Agonists induced neither macroscopic ocular side effects, nor an effect on the pupil size. Possibly, the subclass of α2-adrenoceptor stimulating drugs represent a group of new antiglaucomatous agents.

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.

Correlation between the inhibitory activities of calcium entry blockers on vascular smooth muscle constriction in vitro after K+-depolarisation and in vivo afterα2-adrenoceptor stimulation

SummaryCalcium entry blockers are known to depress the maximal increase in diastolic pressure induced in pithed rats by selective α2-adrenoceptor stimulation. In order to substantiate the relevance of calcium fluxes in this interaction, a correlation between the potency of calcium entry blockers in pithed rats and their calcium antagonistic potency in vitro was investigated. Accordingly, the inhibition of calcium entry blockers with respect to α2-adrenoceptor-mediated vasoconstriction was quantified in terms of −log ID50 values. As a measure of calcium antagonistic potency, the inhibitory activity of a number of calcium entry blockers on K+-induced contractions of rabbit thoracic aorta strips was determined and the negative logarithms of the 50% inhibitory concentrations, −log IC50, were calculated. A highly significant linear relationship was found between the in vitro, −log IC50, and the in vivo parameter, −log ID50, of the calcium entry blockers. This result suggests a similar type of interaction for the calcium entry blockers in vivo and in vitro, i.e. inhibition of a transmembrane influx of extracellular calcium.

Characterization of the angiotensin II-receptor subtype in the longitudinal smooth muscle of the rat portal vein

SummaryThe purpose of the present study was to identify the angiotensin II-receptor subtype involved in the enhancement of the amplitude of the phasic contractions by angiotensin II in the isolated rat portal vein preparation.At an extracellular Ca2+ concentration of 0.9 mmol/l and a K+ concentration of 4 mmol/l, angiotensin II induced concentration-dependent increases in the amplitude of the phasic contractions. The enhancement of phasic contraction amplitude caused by angiotensin II was not significantly altered by pretreatment of the rat portal vein with indomethacin 10−5 mol/l or nitro-L-arginine 10−4 mol/l, indicating that neither prostaglandins nor the endothelium derived-relaxing factor (NO) are involved. Losartan (DuP 753), a nonpeptide selective AT1-receptor antagonist, concentration-dependently shifted the concentration-response curve for the effect of angiotensin II on the amplitude of the contractions to the right, without reducing the maximal response (pA2 = 8.6, slope = 0.98), thus suggesting competitive antagonism at the level of AT1-receptors. By contrast, PD 123177, a nonpeptide selective AT2-receptor antagonist, even at 10−5 mol/l, caused no significant change of the phasic myogenic response to angiotensin II, indicating the absence of AT2-receptor involvement. Dithiothreitol, a disulfide-reducing agent which is known to inactivate AT1-receptors in various tissues, markedly inhibited (3 mmol/l) or even abolished (5 mmol/l) the contractile response of the rat portal vein to angiotensin II, supporting the conclusion that these receptors can be classified as AT1-receptors.In conclusion, the receptor subtype mediating the angiotensin II-induced potentiation of the spontaneous phasic contractions in the rat portal vein appears to belong to the AT1-receptor subtype.

Central and Peripheral α-Adrenoceptors

Summary: The recent interest in the characterization and functional role of α-adrenoceptors has prompted us to study the following different, although interdigitated. lines of research: (a) The functional role of calcium ions in the process of vasoconstriction, induced by α2-adrenoceptor stimulation. We have shown in various animal species that the vasoconstriction induced by α2-adrenoceptor stimulation with specific agonists is impaired by various organic and inorganic calcium antagonists. This finding suggests that the influx of extracellular calcium is required in order to enable the occurrence of vasoconstriction, mediated by the stimulation of vascular α2-adrenoceptors. (b) The functional role of cardiac presynaptic α2-adrenoceptors in the bradycardiac effect of clonidine and related congeners. In the pentobarbitone-anaesthetized rat. the stimulation of cardiac presynaptic α2-adrenoceptors appears to be a highly relevant mechanism, which in combination with the facilitation of the vagal reflex bradycardia explains the bradycardic action of clonidine and related drugs, (c) The possible extrasynaptic location of postsynaptic α2- and β2-adrenoceptors in vascular smooth muscle. A comparison of the influence of the blocking potency of selective α1- and α2-adrenoceptor antagonists on the vasoconstrictor and tachycardic effects of catecholamines, either released endogenously or injected via an exogenous route, has suggested that the α2,- and β2-adrenoceptors are probably located at extrasynaptic sites, whereas the α1-adrenoceptors are rather situated within the synapse, (d) the characterization of α-adrenoceptors involved in the central hypotensive and sedative activities of clonidine. Experiments with selective α-adrenoceptor blocking agents have revealed that the central α-adrenoceptors involved in the hypotensive and sedative effects of clonidine are most likely to correspond to the α2-subtype.

Differential effects of selective 287-1287-1287-1287-2287-2287-2agonists on intraocular pressure in the conscious rabbit

In the present study the influence of topically applied selective α1 and α2-adrenoceptor agonists on intraocular pressure and the diameter of the pupil was investigated in conscious rabbits. Selective stimulation of the α1-subtype of receptors induced an elevation in intraocular pressure, accompanied by mydriasis, whereas stimulation of the α2-subtype caused a marked and dose-dependent ocular hypotensive response, which could be blocked by the selective α2-adrenoceptor antagonist yohimbine. After application of the non-selective α-adrenoceptor agonist clonidine the α2-adrenoceptor mediated decrease in IOP is probably counteracted by a pressor effect due to the stimulation of α1-adrenoceptors.

Calcium dependency of the AT1-receptor mediated effects in the rat portal vein: influence of calcium antagonists

The calcium dependency of AT1-receptor mediated contractions was studied in isolated rat portal vein preparations.The spontaneous phasic contractile force of the rat portal vein was increased (ED50 = 1.76 mmol/l) and the frequency of contractions decreased by raising the extracellular calcium concentration. The Ang 11-induced rise in phasic contractile force (mediated by AT1-receptors, Zhang et al. 1993) proved most pronounced at 0.9 mmol/l of calcium chloride, but it was weaker at either lower or higher calcium concentrations. The maximal increases in the phasic contractile force induced by Ang II were 2.4±0.4, 14.8±0.9 and 5±0.5 mN at calcium concentrations of 0.5, 0.9 and 2.5 mmol/l, respectively. Calcium antagonists reduced at the lower and abolished at the higher concentrations (nifedipine 2×10−8 or 10−7 mol/l; verapamil 10−7 or 5 × 10−7 mol/l; diltiazem 3 × 10−7 or 10–6 mol/l) the spontaneous contractile force. All of these calcium antagonists caused a strong inhibition or suppression of the phasic contractions induced by Ang II.The rank order of potency was nifedipine >verapamil > diltiazem. Ang II (10−6 mol/l) elicited a tonic contraction which was abolished by the AT1-receptor antagonist losartan 10-6 mol/l but not by the AT2-receptor antagonist PD 123177 (10–5 mol/l). Very high concentrations of nifedipine (10–6 mol/l), verapamil (5 × 10-6 mol/l) and diltiazem (5 × 10−6 mol/l) almost suppressed the tonic effect evoked by the activation of AT1-receptors.In a nominally Ca2+ “free”, EGTA-containing solution, a single supra-maximal concentration of Ang II (10−6 mol/l) caused a transient contraction, also mediated by AT1-receptors. This finding suggests the existence of Ang II-sensitive intracellular calcium stores in this preparation. The depletion of such stores proved complete after 4–6 min of perfusion in a Ca2+ “free”, EGTA-containing solution.In conclusion, various types of contractions (a transient contraction in a Ca2+-“free” medium, phasic and tonic contractions) induced by Ang II in the rat portal vein proved to be mediated by AT1-receptors. These contractions were clearly modified by changes in the availability of extra- and possibly intracellular calcium ions. The calcium movements elicited by stimulation of AT1-receptors in a calcium containing solution were inhibited by the three calcium antagonists investigated.

Antihypertensive Activity of the Novel, Nonpeptide Angiotensin II Receptor Antagonists BIBS 39 and BIBS 222 in Conscious Renal Hypertensive Rats

BIBS 39 and BIBS 222 are novel, nonpeptide angiotensin II receptor antagonists with a benzimidazole structure. These compounds, in contrast to the angiotensin II receptor subtype 1 (AT1) selective drug DuP 753, also display affinity for the angiotensin II receptor subtype 2. Their antihypertensive activity was established in conscious renal hypertensive rats. BIBS 39, BIBS 222 and DuP 753, when administered intravenously, caused a substantial dose-dependent antihypertensive effect with very similar ED30 values of approximately 2 mg/kg. The antihypertensive effect was accompanied by mild and transient reflex tachycardia. Captopril, when administered directly after the maximally effective dose of the three angiotensin II receptor antagonists, caused no further fall in blood pressure. This finding indicates that these angiotensin II receptor antagonists had caused a functionally effective suppression of the activity of the renin-angiotensin-aldosterone system. BIBS 222 had a longer duration of antihypertensive action than its congener BIBS 39. The hemodynamic pattern of DuP 753 was somewhat more complex; after a rapid fall and recovery blood pressure was maintained for a long time at a level somewhat below the control value. BIBS 39 and BIBS 222 appear to be effective antihypertensives in the model of the renal hypertensive rat. Their activity is likely to be associated with AT1 receptor blockade.

Vascular aspects of calcium antagonists

Calcium antagonistic drugs have been recognized as useful antiarrhythmic agents for approximately a decade. More recently, however, their vascular effects have been shown to be of potential therapeutic and pharmacological interest, especially in the treatment of angina pectoris involving coronary arterial spasm and possibly also of hypertension. The present survey is dealing with the circulatory changes of the calcium antagonists, describing the influence of the drugs on the systemic and coronary circulation, as well as on special vascular beds like the renal and cerebral circulatory tracts. The influence of the calcium antagonistic agents on vascular smooth muscle at a cellular level is the main subject of the present article. The calcium antagonists inhibit the influx of calcium ions into the cell. Accordingly, they prevent the vasoconstriction, induced by the stimulation of vascularα2-adrenoceptors. However, the calcium antagonists do not blockα2-adrenoceptors as such, but they prevent the stimulation of the contractile proteins. This type of interaction has been demonstrated in various animal species for a variety of calcium antagonists on the one hand and for several selective or non-selectiveα2-adrenoceptor stimulants on the other hand. The vasodilator effect of calcium antagonists may be explained by the selective inhibition of the vasoconstriction, induced by the stimulation ofα2-adrenoceptors by endogenous (nor)adrenaline.