M.-J. Mathy

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.

CENTRAL 5-HT1A RECEPTORS AND THE MECHANISM OF THE CENTRAL HYPOTENSIVE EFFECT OF (+)8-OH-DPAT, DP-5-CT, R28935, AND URAPIDIL

This study investigated the central hypotensive effects of drugs that possess a high affinity for central 5-hydroxytryptamine (5-HT1A) binding sites; (+)8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), N,N-dipropylcarboxamidotryptamine (DP-5-CT), erythro-1-(1-[2(1,4-benzodioxan-2-yl)-2-hydroxyethyl]- 4-piperidyl)-2-benzimidazolinone (R28935) and urapidil proved to possess high affinity and selectivity for central 5-HT1A binding sites, labeled by [3H]8-OH-DPAT. (+)8-OH-DPAT (0.1-10 micrograms/kg) given through the left vertebral artery of chloralose-anesthetized cats, lowered blood pressure by a biphasic dose-response curve. When given systemically, 10- to 100-fold higher doses of (+)8-OH-DPAT were necessary to obtain the same hypotensive effect when compared with central administration. Besides 8-OH-DPAT, R28935, DP-5-CT and urapidil also lowered blood pressure by a central mechanism in doses that were ineffective when given systemically. The central hypotensive effect of 0.3 micrograms/kg (+)8-OH-DPAT, 3 micrograms/kg DP-5 CT, and 3 micrograms/kg R28935 could be blocked by 100 micrograms/kg (-)pindolol, indicating that central 5-HT1A receptors are involved. High doses of (+)8-OH-DPAT (3-10 micrograms/kg) can also lower blood pressure by activating central alpha 2-adrenoceptors. The hypotensive effect of 300 micrograms/kg urapidil given through the vertebral artery could not be blocked by (-)pindolol. These results indicate the involvement of central 5-HT1A receptors in the mechanism of the central hypotensive activity of (+)8-OH-DPAT, DP-5-CT, and R28935.

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.

α1/α2-Adrenoceptor agonist selectivity of mono- and dihydroxy-2-N,N-di-n-propylaminotetralins

The pressor activities and the identity of the postjunctonal α-adrenoceptors involved were determined for a series of congeneric mono- and dihydroxy-substituted 2-N,N-di-n-propylaminotetralins and N,N-di-n-propyldopamine (DPDA) following i.v. administration to pithed normotensive rats. The affinity for α1- and α2-adrenoceptor-like binding sites was obtained from radioligand displacement studies. The 5- and 7-OH substituted tetralins as well as DPDA were reasonably potent and about equieffective pressor agents. The 6-OH congener had almost no vasoconstrictor effects whereas the 8-OH positional isomer occupied an intermediate position. The 5,6- and 6,7-di-OH analogs very effectively raised the diastolic pressure of pithed rats. On account of the inhibition exerted by prazosin (0.1 mg/kg) and yohimbine (1 mg/kg) the 5- and 7-OH isomers as well as DPDA can be classified as mixed α1/α2-adrenoceptor agonists, the α1-adrenoceptor-stimulating potency being more pronounced, especially for the 5-OH congener. In addition, a significant contribution of serotonin receptors to the pressor responses to the 8-OH compound was detected. Similarly, α2-adrenoceptors were mainly responsible for the vasoconstriction caused by the 6,7-di-OH isomer, whereas the 5,6-di-OH congener very selectively stimulated this α2-type receptor in the lower dose range and α1-adrenoceptor stimulation predominated at higher doses of this agonist. The 6,7-di-OH compound failed to activate vascular postjunctional β2-adrenoceptors. The results indicate that the α1/α2-adrenoceptor agonist activity depends on the position(s) and the number of hydroxy groups present as well as on the alkyl substitution at the amino function. 2-N,N-Di-n-propylamino-6,7-dihydroxytetralin may be a more suitable α2-adrenoceptor selective agonist than M-7.

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.

CENTRAL HYPOTENSIVE ACTIVITY OF KETANSERIN IN CATS

Ketanserin, when infused into the left thoracic vertebral artery of chloralose-anaesthetized cats, induced a significantly stronger hypotensive effect than when administered via the systemic circulation. The effect of ketanserin, which is initiated most likely in the pontomedullary region of the brain, proved to be dose dependent. The central hypotensive effect of ketanserin was accompanied by modest bradycardia, which was probably also of central nervous origin. The selective 5-HT2-receptor antagonist ritanserin, when infused into the left vertebral artery, did not lower blood pressure or heart rate, thus suggesting that central 5-HT2 receptors do not play a significant role in the initiation of ketanserins central hypotensive action. Similarly, neither α1- nor α2-adrenoceptors in the central nervous system (CNS) appear to be involved in the hypotensive action of ketanserin, as concluded from pretreat-ment experiments with corynanthine and rauwolscine, respectively. The central hypotensive action could be blocked by atropine. This effect is located within the CNS but does not implicate a direct influence of ketanserin on CNS muscarinic receptors.

Streptozotocin-induced diabetes mellitus in spontaneously hypertensive rats: A pathophysiological model for the combined effects of eypertension and diabetes

Abstract The present study was undertaken to investigate the combined effects of hypertension and streptozotocin-induced diabetes mellitus in the rat. Accordingly, four groups of rats were studied: Wistar Kyoto rats (WKY), diabetic WKY, spontaneously hypertensive rats (SHR) and diabetic SHR, respectively. The mean arterial blood pressure was increased in hypertensive animals compared to normotensive animals. The base excess in the diabetic rats was higher than that of normoglycemic animals. An elevated glucose concentration was found in the blood and urine of streptozotocin-treated rats. Ketone bodies were detected in the urine and blood of the diabetic rats. Mortality rates after treatment were not different among the four groups. In separate experiments, isolated working hearts of the various groups were set up and analyzed. For the maximal left ventricular pressure (mm Hg) the following values were formed: 110.0 ± 2.6, 93.6 ± 2.7, 93.4 ± 3.0, and 87.5 ± 2.4, respectively. The wet heart weights, dry heart weights, and body weights of the diabetic rats were lower than those of normoglycemic animals. The wet heart weight/body weight ratio, however, was increased by diabetes and hypertension (0.43 ± 0.01, 0.47 ± 0.01, 0.47 ± 0.01, and 0.54 ± 0.02, respectively). There were no significant differences between the water content of the hearts from the four different groups. Pathologic examination of the hearts showed myocardial hypertrophy and medial hypertrophy of coronary arteries in diabetic and hypertensive animals. There was no difference in relative collagen content in the hearts of the four groups. In conclusion, the experimental procedure used appears to be an unique model to study the effects of diabetes and hypertension separated as well as combined.

INTERACTIONS BETWEEN THE PUTATIVE CALCIUM ENTRY PROMOTOR BAY-K-8644 AND PRESSOR-RESPONSES PRODUCED BY ALPHA-1-ADRENOCEPTOR AND ALPHA-2-ADRENOCEPTOR AGONISTS IN THE PITHED NORMOTENSIVE RAT

SummaryInteractions between the putative calcium entry promotor Bay k 8644 and both α-1 and α1-adrenocepter mediated increases in diastolic pressure were studied in the pithed normotensive rat. The α1-adrenocepter mediated pressor responses elicited by B-HT920, TL-99, DP-6,7-ADTN and B-HT958 were potentiated by Bay k 8644, reflected by a leftward shift and an increase in the maximum of the log dose-pressor respinse curves. The α-1-adrenocepter effects elicited by cirazoline, methoxamine, (−)-amidephrine, St 587, (−)-phenylephrine and Sgd 101/75 were less enhanced by Bay k 8644. Only a leftward shift of the dose-response curves was observed, which was most pronounced for (−)-phenylephrine and Sgd 101/75. The α-1 and α2-adrenocepter pressor components of (−)-noradrenaline were similarly distinguished by Bay k 8644 as observed for the selective α-1 or α2-adrenocepter agonists.Effects of Bay k 8644 on the increase in diastolic pressure mediated by B-HT 920, St 587 and cirazoline were also studied after pretreatment with the calcium entry blocker nifedipine. After additional pretreatment with nifedipine the potentiation by Bay k 8644 observed for B-HT 920 and St 587 was more pronounced. The presence of nifedipine had no effect on the interaction between Bay k 8644 and cirazoline.It is concluded that Bay k 8644 behaves as a mirror image of nifedipine. The observation that Bay k 8644 enhances α2-adrenocepter mediated pressor effects more effectively than α1-adrenocepter increases in diastolic pressure is in accordance with the hypothesis of the more pronounced calcium dependency of α2-adrenocepter mediated pressor responses. The data obtained for ceptor mediated pressor responses. The data obtained for St 587 and (−)-phenylephrine are in apparent contradiction to the finding that the pressor responses to the former drug are more markedly inhibited by calcium entry blockade than those of the latter. It is suggested that St 587 employs calcium channels which are already maximally modulated and that (−)-phenylephrine makes use of calcium channels which are in a rather inactive state. The hypothesis is put forward that the intrinsic activity of α2-adrenocepter agonists reflects their ability to bring calcium channels in an active state.

Influence of respiratory acidosis or alkalosis on pressor responses mediated byα1- andα2-adrenoceptors in pithed normotensive rats

SummaryThe effect of respiratory acidosis and alkalosis on the vasoconstriction toα1- andα2-adrenoceptor stimulation was studied in pithed normotensive rats. The selectiveα1-adrenoceptor agonists (-)amidephrine, cirazoline, (±)erythro methoxamine (-)phenylephrine, Sgd 101/75 and St 587 were used, as well as the selectiveα2-adrenoceptor agonists B-HT 920, B-HT 933, DP-6,7-ADTN, M-7 and UK 14,304. The non-selectiveα-adrenoceptor agonists xylazine, noradrenaline and adrenaline were included as well. The latter two were also studied under selective doses of the antagonists rauwolscine and prazosin, thus yielding the respectiveα1- andα2-adrenoceptor components of the vasoconstriction to these agonists. The effect of acid-base balance disturbances on presynaptically released noradrenaline elicited by electrical stimulation of preganglionic nerves was studied as well. Dose response curves for the agonists were generated under various conditions of ventilation, yielding either alkalotic, normal or acidotic values of arterial blood pH. Pressor responses to all agonists were maximally affected by changes in acid-base status at the low doses of the agonists. Acidosis was found to inhibit increases in diastolic pressure mediated by theα1- as well as theα2-adrenoceptor agonists studied, although not to the same extent. Alkalosis exerted either an obvious potentiation or did not significantly influenceα1-adrenoceptor mediated pressor responses. On the basis of acid-base sensitivity the following groups of agonists were distinguised: (1) Cirazoline, phenylephrine, methoxyamine, electrically released noradrenaline from presynaptic sites, of which pressor responses are obviously potentiated and attenuated by alkalosis and acidosis, respectively. (2) Sgd 101/75, St 587, noradrenaline-α1, amidephrine and adrenaline-α1, eliciting pressor responses which are strongly acid-sensitive and base-insensitive. (3) B-HT 920, B-HT 933, DP-6, 7-ADTN (lower doses), of which vasoconstriction is markedly inhibited by both acidosis and alkalosis. (4) Noradrenaline-α2, UK 14,304, M-7 (lower doses), adrenaline-α2 and high doses of the agonists of the former group. Pressor responses of these agonists were found to be not or slightly base-sensitive, but profoundly acid-sensitive. Xylazine does not fit into this classification. The present data are not in accordance with purported subdivisions ofα-adrenoceptor agonists by others. It is therefore concluded that the differential effect of acid-base status onα-adrenoceptor mediated vasoconstriction in pithed rats is an exponent of the differential way of interaction of the agonists with the adrenoceptors involved.

α1-adrenoceptor-mediated vasoconstriction in vivo to enantiomers of SK&F 89748-A

The pressor activity of the 1-enantiomer of SK & F 89748-A, 1,2,3,4-tetrahydro-8-methoxy-5-(methylthio)-2-naphthalenamine, in pithed normotensive rats was found comparable with that of 1-phenylephrine. The d-enantiomer was half as potent. The log dose-pressor effect curves for d- and 1-SK & F 89748-A were not influenced by reserpine treatment (2 X 5 mg/kg i.p., -48 and -24 h), were virtually unaffected by yohimbine (1 mg/kg i.v., -15 min) but were markedly shifted to the right by prazosin (0.1 mg/kg i.v., -15 min) and phentolamine (1 mg/kg i.v., -15 min). Similar observations were made for the 1-enantiomer in pithed cats. It is concluded that d- and 1-SK & F 89748-A are potent, directly acting highly selective agonists of (vascular) postjunctional alpha 1-adrenoceptors. Potency and selectivity were equally pronounced for both enantiomers. The currently available selective agonists of alpha 1-adrenoceptors, including the optical isomers of SK & F 89748-A, cannot distinguish between alpha 1- and alpha 2-adrenoceptors. This conclusion is based on binding affinity since these affinities are linearly correlated as shown by radioligand displacement experiments.