Mechanisms of augmented vasoconstriction induced by 5-hydroxytryptamine in aortic rings from spontaneously hypertensive rats

Abstract

Background and purpose: To test whether development of enhanced vasoconstriction to 5-hydroxytryptamine (5-HT; serotonin) in SHR was temporally related to hypertension, elevated vascular superoxide (O(2)(-)) levels, decreased NO bioavailability, or increased contractile effects of cyclooxygenase or rho-kinase and/or PKC.

Experimental approach: We examined systolic blood pressure (SBP), vascular O(2)(-), and 5-HT-induced contractile responses of aortic segments from 4- and 8-week-old WKY and SHR.

Key results: SBP was 35% higher in SHR than WKY at 4 weeks and 60% higher at 8 weeks. Contractile responses to 5-HT were similar in WKY and SHR at 4 weeks, but were markedly augmented in SHR at 8 weeks. The NO synthase inhibitor, L-NAME, enhanced contractile responses to 5-HT markedly in both strains at 4 weeks and in WKY at 8 weeks, but only very modestly in SHR at 8 weeks. These functional differences were associated with higher O(2)(-) levels in SHR versus WKY at 8 weeks, but not at 4 weeks. The rho-kinase inhibitor, Y-27632, and the PKC inhibitor, Ro 31-8220, each only modestly attenuated contractions in WKY and SHR in each age group, and their effects in each strain were more pronounced at 8 weeks. The cyclooxygenase inhibitor, indomethacin, had no effect on contractile responses.

Conclusions and implications: Development of augmented vascular contractile responses to 5-HT in SHR is preceded by hypertension. It is associated with increased vascular O(2)(-) levels and reduced modulatory effects of NO, and is unlikely to be due to enhanced activity of rho-kinase, PKC or cyclooxygenase.

Figure 1

Systolic blood pressure and aortic O₂⁻ levels in WKY and SHR. (a) Measurement of systolic blood pressure in WKY and SHR at 4 and 8 weeks. (b) O₂⁻ levels in aortic segments from WKY and SHR at 4 and 8 weeks. All values are mean±s.e.mean. n=8 of each strain per group. ^*P<0.05, unpaired t-test versus WKY, ^†P<0.05, unpaired t-test versus 4 weeks. SHR, spontaneously hypertensive rats; WKY, wistar–Kyoto.

Figure 2

Contractile responses to 5-HT in WKY and SHR. Concentration–response curves to 5-HT in aortic segments from (a) 4-week- and (b) 8-week-old WKY and SHR. All values are mean±s.e.mean. n=15–16 of each strain per group. ^*P<0.05 versus WKY maximum, ^⊘P<0.05 (unpaired t-test) versus WKY log EC₅₀. SHR, spontaneously hypertensive rats; WKY, wistar–Kyoto.

Figure 3

Effect of NOS inhibition on contractile responses to 5-HT. Concentration–response curves to 5-HT in aortic segments from (a and c) WKY and (b and d) SHR at 4 and 8 weeks of age, in the presence of vehicle (Veh; saline) or L-NAME (100 μM). All values are mean±s.e.mean. n=8 per group. ^*P<0.05, unpaired t-test versus Veh maximum, ^⊘P<0.05, unpaired t-test versus Veh log EC₅₀. NOS, nitric oxide synthase; SHR, spontaneously hypertensive rats; WKY, wistar–Kyoto.

Figure 4

Effect of cyclooxygenase inhibition on contractile responses to 5-HT. Concentration–response curves to 5-HT in aortic segments from (a) WKY and (b) SHR at 8 weeks of age, in the presence of vehicle (Veh; 1 mM Na₂CO₃) or indomethacin (10 μM). All values are mean ±s.e.mean. n=7–8 per group.

Figure 5

Effects of rho-kinase and PKC inhibition on contractile responses to 5-HT. Concentration–response curves to 5-HT in aortic segments from (a and c) WKY and (b and d) SHR at 4 and 8 weeks of age, in the presence of vehicle (Veh; 0.5% dimethyl sulphoxide), Ro 31-8220 (Ro, 5 μM) or Y-27632 (Y, 1 μM) alone, or in combination. All values are mean±s.e.mean. n=7–8 per group. ^*P<0.05 versus Veh maximum, ^†P<0.05 versus Y-27632 maximum; one-way ANOVA followed by Tukey's test. SHR, spontaneously hypertensive rats; WKY, wistar–Kyoto.