Alpha1-adrenoceptor subtypes involved in vasoconstrictor responses to exogenous and neurally released noradrenaline in rat femoral resistance arteries

Abstract

1. The alpha(1)-adrenoceptor subtypes involved in responses to exogenous and neurally released noradrenaline in rat femoral resistance arteries were characterised using a small vessel myograph, with antagonists prazosin (nonsubtype selective), 5-methyl-urapidil (alpha(1A)-selective), BMY 7378 (alpha(1D)-selective) and the alkylating agent chloroethylclonidine (preferential for alpha(1B)-). 2. Prazosin and 5-methyl-urapidil produced rightward shifts of the exogenous noradrenaline concentration - response curve (CRC) with pA(2) values of 9.2 and 9.1 respectively, in agreement with the presence of alpha(1A)-adrenoceptors. BMY 7378 (1 microm) shifted the noradrenaline CRC with an apparent pK(B) of 6.7, in agreement with the presence of alpha(1A)-, but not alpha(1D)-, adrenoceptors. Chloroethylclonidine at 1 microm had no effect and at 10 microm produced only a small reduction (c. 20%) in the maximum response to noradrenaline, indicating little, if any, contribution from alpha(1B)-adrenoceptors. 3. Responses of the rat femoral resistance arteries to electrical field stimulation (EFS) at 5-30 Hz for 10 s and 0.05 ms pulse width were principally due to alpha(1)-adrenoceptor stimulation. Prazosin and 5-methyl-urapidil inhibited EFS-mediated responses with pIC(50)s of 9.3 and 8.2, respectively, consistent with the alpha(1A)-adrenoceptor being the predominant subtype. Responses to EFS at 10-30 Hz were relatively insensitive to BMY 7378 (pIC(50), 6.5-6.7), while responses to 5 Hz were inhibited with a significantly higher pIC(50) of 8.02, suggesting the contribution of alpha(1D)-adrenoceptors. Chloroethylclonidine had no effect on responses to EFS, ruling out the contribution of an alpha(1B)-subtype. In the presence of cocaine, the predominant subtype involved in responses to EFS was the alpha(1A)-adrenoceptor, with a contribution from alpha(1D)-adrenoceptors at low frequency, as seen in the absence of cocaine. However, there was also a significant increase in the sensitivity to BMY 7378 at higher frequencies, suggesting that a further small alpha(1D)-adrenoceptor component may be uncovered in the presence of cocaine. 5. The present study has shown a predominant role of the alpha(1A)-adrenoceptor in contractions due to exogenous noradrenaline and to neurally released noradrenaline in rat femoral resistance arteries. alpha(1D)-Adrenoceptors are not involved in responses to exogenous noradrenaline but appear to be activated by neurally released noradrenaline at a low frequency of stimulation and at higher frequencies in the presence of neuronal-uptake blockade.

Figure 1

Effects of antagonists on responses of rat femoral resistance arteries to exogenous noradrenaline; (a) prazosin (n=9); (b) 5-methyl-urapidil (5 MU) (n=6); (c) BMY 7378 (BMY) (n=5); (d) chloroethylclonidine (CEC) (n=4).

Figure 2

Effects of drugs on the responses of rat femoral resistance arteries to electrical field stimulation at different frequencies for 10 s and 0.05 ms pulse width. (a) Tetrodotoxin (TTX, 1 μM) (n=5); (b) guanethidine (GUAN, 10 μM) (n=4); (c) RS 79488 (0.1 μM) (n=4). Significance of difference from control, ^*P<0.05, ^**P<0.01, ^***P<0.001 (paired t-test.)

Figure 3

Effects of prazosin and α, β-methylene ATP on the responses of rat femoral resistance arteries to electrical field stimulation at different frequencies for 10 s and 0.05 ms pulse width. (a) Prazosin (0.1 μM) followed by α, β-methylene ATP (10 μM) (n=5); (b) α,β-methylene ATP (10 μM) followed by prazosin (0.1 μM) (n=4). Significance of difference from control, ^*P<0.05, ^**P<0.01, ^***P<0.001 (paired t-test.)

Figure 4

(a) Effect of different concentrations of prazosin on responses of rat femoral resistance arteries to electrical field stimulation at different frequencies for 10 s and 0.05 ms pulse width. Significances of difference from control have been omitted for clarity. All treatment values were significantly different from control (P<0.001) except for 0.1 nM prazosin (P>0.5) at all frequencies (repeated measures ANOVA with post tests, n=8). (b) Effect of different concentrations of 5-methyl-urapidil (5 MU) on responses of rat femoral resistance arteries to electrical field stimulation at different frequencies for 10 s and 0.05 ms pulse width. Significances of difference from control have been omitted for clarity. All treatment values were significantly different from control (P<0.001) except for 1 nM 5 MU (P>0.5) at all frequencies, (repeated measures ANOVA with post-tests, n=5).

Figure 5

(a) Effect of BMY 7378 on responses of rat femoral resistance arteries to electrical field stimulation at different frequencies for 10 s and 0.05 ms pulse width (n=5). Significance of difference from control, ^**P<0.01, ^***P<0.001 (repeated measures ANOVA followed by post-tests. (b) Effect of BMY 7378 on responses of rat femoral resistance arteries to electrical field stimulation at different frequencies for 10 s and 0.05 ms pulse width in the presence of cocaine, 3 μM (n=7). Significance of difference from control, ⁺P<0.05, ⁺⁺P<0.01, ⁺⁺⁺P<0.001 (paired t-test). Significance of difference from cocaine, ^***P<0.001 (repeated measures ANOVA with post-test).