Differential response to chloroethylclonidine in blood vessels of normotensive and spontaneously hypertensive rats: role of alpha 1D- and alpha 1A-adrenoceptors in contraction

Abstract

The effects of chloroethylclonidine on alpha(1)-adrenoceptor-mediated contraction in endothelium-denuded caudal arteries and aorta from normotensive Wistar and Wistar Kyoto (WKY), and from spontaneously hypertensive (SHR) rats were evaluated. Chloroethylclonidine elicited concentration-dependent contractions. Maximal contraction was similar in caudal arteries among strains ( approximately 40% of noradrenaline effect). However, chloroethylclonidine elicited a higher contraction in aorta from SHR than from normotensive rats. In Wistar aorta chloroethylclonidine produced the smallest contractile response. In SHR aorta, BMY 7378 and 5-methylurapidil blocked chloroethylclonidine-elicited contraction, while (+)-cyclazocine did not inhibit it; while in caudal arteries, 5-methylurapidil blocked chloroethylclonidine action; the other antagonists had no effect. In chloroethylclonidine-treated aorta noradrenaline elicited biphasic contraction-response curves, indicating a high affinity (pD(2), 8.5 - 7.5) chloroethylclonidine-sensitive component and a low affinity (pD(2), 6.3 - 5.2) chloroethylclonidine-insensitive component. The high affinity component was blocked by chloroethylclonidine; while in caudal arteries noradrenaline elicited monophasic contraction-response curves with pD(2) values (6.5 - 5.7) similar to the low affinity component in aorta. Chloroethylclonidine inhibition of noradrenaline response was greater in aorta than in caudal arteries. Chloroethylclonidine increased the EC(50) values of noradrenaline approximately 1000 fold in aorta and approximately 10 fold in caudal arteries. In SHR aorta BMY 7378 protected alpha(1D)-adrenoceptors and in caudal arteries 5-methylurapidil protected alpha(1A)-adrenoceptors from chloroethylclonidine alkylation, allowing noradrenaline to elicit contraction. These results show marked strain-dependent differences in the ability of chloroethylclonidine to contract aorta; moreover, chloroethylclonidine stimulates alpha(1D)-adrenoceptors in aorta and alpha(1A)-adrenoceptors in caudal arteries. The higher contraction observed in aorta from SHR and WKY suggests an augmented number of alpha(1D)-adrenoceptors in these strains.

Figure 1

Concentration-response of chloroethylclonidine-induced contraction in caudal arteries from Wistar, WKY and SHR rats. Arterial rings were incubated with (±)-propranolol and rauwolscine (100 nM, each) prior to and during chloroethylclonidine (1–100 μM) exposure. Contraction is expressed as percentage of maximal noradrenaline effect (2.36±0.09, 1.15±0.05 and 0.73±0.04 g for Wistar, WKY and SHR, respectively). Results represent the means±s.e.mean of 4–6 different animals.

Figure 2

Effect of 5-methylurapidil, (+)-cyclazocine and BMY 7378 on chloroethylclonidine-induced contraction in caudal arteries of spontaneously hypertensive rats. Caudal rings were incubated with BMY 7378, (+)-cyclazocine or 5-methylurapidil prior to and during chloroethylclonidine 1–100 μM. Results represent the means± s.e.mean of three different animals. ^*P<0.05 vs chloroethylclonidine.

Figure 3

Concentration-response of chloroethylclonidine-induced contraction in aorta from Wistar, WKY and SHR rats. Aortic rings were incubated with (±)-propranolol and rauwolscine (100 nM, each) prior to and during chloroethylclonidine (1–100 μM) exposure. Data shows the percentage of maximal noradrenaline effect (1.74±0.04, 1.01±0.04, 0.96±0.03 and g for Wistar, WKY and SHR, respectively), and represent the means±s.e.mean of 4–6 different animals. +P<0.05 vs WKY; ^*P<0.05 vs Wistar.

Figure 4

Effect of BMY 7378, 5-methylurapidil and (+)-cyclazocine on chloroethylclonidine-induced contraction in aorta from spontaneously hypertensive rats. Aortic rings were incubated with BMY 7378, 5-methylurapidil and (+)-cyclazocine prior to and during chloroethylclonidine (1–100 μM) exposure. Results represent the means±s.e.mean of 3–4 different animals. ^*P<0.05 vs chloroethylclonidine.

Figure 5

Effect of chloroethylclonidine on the concentration-response curves to noradrenaline in caudal arteries from Wistar, WKY and SHR rats. Arterial rings were incubated with increasing concentrations of chloroethylclonidine, after 45 min arteries were extensively washed and then subjected to a concentration-response curve to noradrenaline. Results are expressed as percentage of maximal noradrenaline-induced contraction and represent the means±s.e.mean of 4–6 different animals. The effect of 1 μM chloroethylclonidine is omitted for clarity.

Figure 6

Effect of chloroethylclonidine on the concentration-response curve to noradrenaline in aorta from Wistar, WKY and SHR rats. Aortic rings from Wistar, WKY or SHR rats were incubated with increasing concentrations of chloroethylclonidine, after 45 min vessels were thoroughly washed and then subjected to a concentration-response curve to noradrenaline. Results represent the means±s.e.mean of 4–6 different animals. The effect of 30 μM chloroethylclonidine is omitted for clarity.

Figure 7

Protection by BMY 7378 of noradrenaline-induced contraction after chloroethylclonidine treatment in aorta from spontaneously hypertensive rats. Aortic rings were incubated with BMY 7378 prior to and during chloroethylclonidine exposure, then subjected to a concentration-response curve to noradrenaline. Data represent the means±s.e.mean of 4–6 different animals. ^*P<0.05 vs chloroethylclonidine.

Figure 8

Protection by 5-methylurapidil and BMY 7378 of noradrenaline-induced contraction after chloroethylclonidine treatment in caudal arteries from spontaneously hypertensive rats. Caudal rings were incubated with 5-methylurapidil or with BMY 7378 before chloroethylclonidine exposure, then subjected to a concentration-response curve to noradrenaline. Data represent the means±s.e.mean of three different animals. ^*P<0.05 vs chloroethylclonidine.