Responses to noradrenaline of portal vein strips from normotensive and spontaneously hypertensive rats

The T- and L-type calcium channel blocker (CCB) mibefradil attenuates leg edema induced by the L-type CCB nifedipine in the spontaneously hypertensive rat: a novel differentiating assay

Among the L-type calcium channel blockers (CCBs), particularly dihydropyridines like nifedipine [1,4-dihydro-2,6-dimethyl-4-(2-nitrophenyl)-3,5-pyridinedicarboxylic acid dimethyl ester], a common adverse effect is vasodilatory edema. Newer CCBs, such as the T- and L-type CCB, mibefradil [(1S,2S)-2-[2[[3-(2-benzimidazolylpropyl]methylamino]ethyl]-6-fluoro-1,2,3,4-tetrahydro-1-isopropyl-2-naphthyl methoxyacetate dihydrochloride hydrate], demonstrate antihypertensive efficacy similar to that of their predecessors but seem to have a reduced propensity to cause edema. Using a magnetic resonance imaging (MRI) T(2) mapping technique, we investigated the ability of mibefradil to reduce extracellular water accumulation caused by the L-type CCB, nifedipine, in the hindleg skeletal muscle of the spontaneously hypertensive rat. Mibefradil (10 mg/kg i.v.) and nifedipine (1 mg/kg i.v.) lowered mean arterial blood pressure by 97 +/- 5 and 77 +/- 4 mm Hg, respectively. MRI edema index (expressed as percentage increase of integral T(2) over predrug control) was significantly higher with nifedipine (2606 +/- 86%; p < 0.05) than with mibefradil (981 +/- 171%) measured 30 to 60 min after the start of drug infusion. The hindleg edema caused by nifedipine was dose dependently decreased by coadministration of mibefradil (0, 0.3, or 3 mg/kg). The hindleg edema formation was not due to albumin leakage into the interstitial space based on immunostaining. However, a 4.2-fold increase in the arterial L-/T-type CC mRNA expression ratio was observed compared with the venous L/T ratio as shown by quantitative reverse transcription polymerase chain reaction. These results demonstrate the novel utility of MRI to measure extravascular water after acute exposure to CCBs and indicate that T-type CCB activity may reduce L-type CCB-induced vasodilatory edema in the skeletal muscle vasculature, possibly by a differential effect on arteriole and venule dilatation.

The role of alpha 1-adrenoceptor subtypes in the phasic and tonic responses to phenylephrine in the longitudinal smooth muscle of the rat portal vein

The purpose of this investigation was to determine whether alpha 1-adrenoceptor subtypes (co)exist in the rat portal vein and, if so, whether they could be functionally associated with the phasic and tonic types of contraction as a response to alpha 1-adrenoceptor stimulation by phenylephrine. A low Ca2+ concentration (0.9 mmol/l) in the Tyrode solution enabled us to quantify changes both in the phasic myogenic activity and in the basal tone of the rat portal vein preparation very precisely. We used both competitive and non-competitive alpha-adrenoceptor antagonists which have been employed successfully by other investigators to discriminate between alpha 1-adrenoceptor subtypes in vascular and other tissues. Schild analysis showed that the competitive alpha-adrenoceptor antagonists prazosin, phentolamine, yohimbine, corynanthine, idazoxan, rauwolscine and 5-methyl-urapidil could not distinguish between the phasic and tonic responses to phenylephrine and/or different alpha 1-adrenoceptor subtypes in the rat portal vein. However, when we compared our pA2 values with those found to be representative indicators according to subclassifications based on the use of selective antagonists in different tissues, the alpha 1-adrenoceptors in the rat portal vein appeared to belong to the alpha 1L- or alpha 1a-subtype. This subclassification was not in accordance with the data obtained with the irreversible alpha-adrenoceptor antagonist chloroethylclonidine. However, the validity of this alkylating agent as a tool for receptor classification was restricted, at least in the rat portal vein, by its effects on receptor reserve.(ABSTRACT TRUNCATED AT 250 WORDS)

Vessel reactivity and prejunctional modulatory changes in the portal vein of mature spontaneously hypertensive rats

The prejunctional effects of 2-chloroadenosine on the contractile responses to perivascular nerve stimulation were studied in conjunction with vessel reactivity in portal veins from mature (45-60 weeks) spontaneously hypertensive rats (SHR) and age-matched Wistar-Kyoto (WKY) controls. It was found that the contractility of the portal veins to exogenous noradrenaline was enhanced in SHR, while the sensitivity to noradrenaline, as seen from the EC50 values, was not altered. Responses to perivascular nerve stimulation (supramaximal voltage, 0.7 ms pulse duration for 10 s) were also enhanced in spontaneously hypertensive rats compared to WKY controls at all frequencies of stimulation tested (2-64 Hz), although the sensitivity to perivascular nerve stimulation was not changed since a response of 50% of the maximal obtainable was achieved at 8 Hz in both SHR and WKY. However, no evidence for change in the level of prejunctional modulation of nerve stimulation responses or postjunctional modulation of noradrenaline responses by 2-chloroadenosine (0.1-100 microM) was found. In conclusion, in mature SHR, responses to perivascular nerve stimulation and exogenous noradrenaline were found to be enhanced compared to responses in WKY, although there was no difference in the modulatory action of 2-chloroadenosine between SHR and WKY.

Prostaglandin I2 and thromboxane A2 production in relation to alpha 1 and alpha 2-adrenoreceptor activation in the normotensive and hypertensive rat

1. The perfused mesenteric arterial bed (MAB) from normotensive (WKY) and hypertensive (SHR) rats was used as a model to investigate the relationship between alpha 1 and alpha 2-adrenoreceptor activation and prostanoid release. 2. Prostaglandin I2 (PGI2) and small amounts of thromboxane A2 (TXA2) are released from this preparation and can be monitored in the perfusate under basal and stimulated conditions. 3. Noradrenaline (NA) caused a significant increase in the release of PGI2 from the MAB of both WKY and SHR. Both basal and NA-stimulated PGI2 release were lower in the SHR, whereas pressor responses to NA were higher. Noradrenaline caused a very small increase in TXA2 release in the WKY only and the basal release of this prostanoid was again lower in the SHR. 4. Prazosin, an alpha 1-selective antagonist (10(-10), 10(-9), 10(-8) M) was more effective in blocking pressor responses to NA in the SHR than in the WKY, did not affect the NA-induced release of PGI2 in either group and abolished the small increase of TXA2 release in WKY. 5. Rauwolscine, an alpha 2-selective antagonist (10(-8), 10(-7), 10(-6) M) was more effective in reducing pressor responses to NA in the SHR than in the WKY. It abolished, from 10(-7) M upwards, the NA-induced release of PGI2 in the WKY and reduced it in the SHR. The NA-induced release of TXA2 was abolished in WKY. 6. The observed differences in the effects of rauwolscine and prazosin do not correlate with the effects of these antagonists on pressor responses since both reduce the latter but only rauwolscine abolishes the stimulated PGI2 release. 7. These results indicate that in the SHR, the mechanisms mediating the release of PGI2 from the MAB may be similar but not identical to those in the WKY. The observation that an increase in adrenergically induced pressor responses is not followed by an increase in PGI2 release, which is in fact reduced in the SHR, suggests a more complex regulation of this relationship which may have pathophysiological implications.

Non-competitive antagonism of the alpha-adrenoceptor-mediated fast component of contraction of rat aorta, by doxazosin and prazosin

alpha-Adrenoceptor antagonists have been compared for their effects on dose-response curves of fast and slow components of contraction of the rat aorta to noradrenaline (NA). All agents caused a competitive antagonism of the slow component of contraction to NA. The order of potency was: prazosin greater than WB4101 = doxazosin greater than tiodazosin greater than phentolamine greater than corynanthine greater than trimazosin greater than rauwolscine. For the fast component, doxazosin, prazosin, tiodazosin and WB4101 caused some depression of the maximum response. Doxazosin (25 nM) and prazosin (25 nM) produced a complete and unsurmountable antagonism of the maximum fast component. Phentolamine, corynanthine, trimazosin and rauwolscine all competitively antagonized the fast component. The degree of antagonism of the fast component by prazosin and its analogues appeared to be directly related to the potency of individual agents for the slow component. WB4101, which was equipotent with doxazosin and more potent than tiodazosin was less effective than either in reducing the fast component. The antagonism of the fast component by prazosin or doxazosin was easily reversed by washing and prevented by phentolamine (2.5 microM). Neither prazosin nor doxazosin in concentrations of up to 2.5 microM has any effect on contractions of the aorta to 5-hydroxytryptamine (5-HT, 0.25-250 microM) or caffeine (20mM). It is concluded that the ability of some alpha-adrenoceptor antagonists to produce a non-competitive antagonism of the fast component of contraction is (a) dependent upon blockade of alpha-adrenoceptors; (b) unrelated to selectivity for alpha 1-adrenoceptors; (c) related to potency and structure. 8 EGTA (3.0 mM) caused a selective suppression of the slow component of contraction to NA. Both doxazosin and prazosin caused a non-competitive antagonism of EGTA-resistant contractions to NA whereas corynanthine showed competitive antagonism. These observations, together with those above imply that prazosin and doxazosin non-competitively antagonize alpha-adrenoceptorinduced release of calcium in the rat aorta, but competitively antagonize alpha-adrenoceptor-induced calcium entry.

Effects of diltiazem on phasic and tonic activity in rat portal vein

The effects of the Ca2+ entry blocking drug, diltiazem, have been evaluated in the rat isolated portal vein, against phasic or tonic responses induced by a range of agonists. Diltiazem was a potent antagonist of phasic responses induced by low concentrations of K+, tetraethylammonium (TEA), the selective alpha 2-adrenoreceptor agonists UK 14304 or TL99 and angiotensin II (AII). Diltiazem was significantly less potent as an antagonist of phasic responses induced by the selective alpha 1-adrenoreceptor agonists phenylephrine (PE) or methoxamine (ME) or the non-selective alpha-adrenoreceptor agonist (NA), or of tonic responses evoked by high concentrations of K+, or PE. The non-stimulated phasic activity of the portal vein was antagonised by diltiazem at higher concentrations only. It is concluded that in the rat portal vein, phasic or tonic activity are associated with different Ca2+-gating mechanisms. It is considered that these differences could represent different Ca2+-channels, different rates of activation or deactivation of the channels, or involve other sources of activator Ca2+ than extracellular Ca2+. The alpha 2-adrenoreceptor subtype may be functionally linked with a voltage dependent Ca2+-channel to cause phasic responses in this preparation.

alpha-adrenoreceptor-mediated phasic and tonic activity in rat portal vein in vitro

The effects of alpha 1- and alpha 2-adrenoreceptor agonists on phasic or tonic activity have been examined in rat portal vein in vitro. Noradrenaline (NA) and Phenylephrine (PE) increased phasic activity at low concentrations, these effects were superimposed on a sustained contracture at higher concentrations. The alpha 2-adrenoreceptor agonists UK 14304 or TL 99 increased extracellular electrical activity and associated mechanical phasic activity without inducing sustained contracture. These alpha 2-adrenoreceptor mediated effects were particularly sensitive to changes in Ca2+ concentration. Prazosin was a potent, competitive antagonist of contracture to NA, or PE, but did not antagonise the phasic responses to NA, UK 14304 or TL 99, except at high concentrations. Prazosin was significantly less potent against PE-induced phasic responses than PE-induced contracture. The results suggest that alpha 1-adrenoreceptors are predominantly involved in contracture. Yohimbine was significantly more potent against UK 14304 or TL 99-induced phasic response than against PE-induced responses, however, phasic responses to NA were competitively antagonised only by combination of yohimbine and prazosin. It is concluded that postsynaptic alpha 2-adrenoreceptor mechanisms are involved in the phasic responses to selective alpha 2-adrenoreceptor agonists, and to NA in rat portal vein. The response of contracture is mediated through alpha 1-adrenoreceptors.

Stereoselective and non-stereoselective effects of D 600 (methoxyverapamil) in smooth muscle preparations

Mechanical responses in guinea-pig ileal longitudinal, rat vasa deferentia and rat portal vein smooth muscle strips elicited by receptor activation (muscarinic or alpha-adrenergic) or by K+ depolarization were blocked by D 600 (methoxyverapamil). Stereoselectivity was observed with the (-)-enantiomer being more potent than the (+)-enantiomer (the ratio varying from 6 to 180). The tonic (slow) component of response was more sensitive than the phasic (fast) component. D 600 competitively blocked binding of (-)-[3H]QNB and [3H]WB 4101 to muscarinic and alpha-adrenergic receptors respectively, but in marked contrast to the effects on mechanical responses antagonism of ligand binding was non-stereoselective. It is suggested that stereoselectivity of action of D 600 may be a useful criterion to distinguish between its several sites of action.