Investigation of mechanisms that mediate reactive hyperaemia in guinea-pig hearts: role of K(ATP) channels, adenosine, nitric oxide and prostaglandins

Abstract

1. Reactive hyperaemia is a transient vasodilatation following a brief ischaemic period. ATP-dependent K(+) (K(ATP)) channels may be important in mediating this response, however it is unclear whether mitochondrial K(ATP) channels contribute to this in the heart. 2. We examined the involvement of K(ATP) channels and the relative role of mitochondrial channels as mediators of coronary reactive hyperaemia and compared them to mechanisms involving NO, prostaglandins and adenosine in the guinea-pig isolated heart. 3. Reactive hyperaemic vasodilatation (peak vasodilator response and flow debt repayment) were assessed after global zero-flow ischaemia (5 -- 120 s) in the presence of nitro-L-arginine methyl ester (L-NAME, 10(-5) M, n=9), 8-phenyltheophylline (8-PT, 10(-6) M, n=12) and indomethacin (10(-5) M, n=12). 4. Glibenclamide (10(-6) M, n=12) a non-selective K(ATP) channel inhibitor and 5-hydroxy-decanoic acid (5-HD, 10(-4) M, n=10) a selective mitochondrial K(ATP) channel inhibitor were also used. The specificity of the effects of glibenclamide and 5-HD (n=6 each) were confirmed using pinacidil (38 nmol -- 10 micromol) and diazoxide (42 nmol -- 2 micromol). Glibenclamide was most effective in blocking the hyperaemic response (by 87%, P<0.001) although 5-HD and 8-PT also had a marked effect (40% inhibition, P<0.001 and 32%, P<0.001, respectively). L-NAME and indomethacin had little effect. 5. Perfusion with L-NAME and glibenclamide significantly reduced baseline coronary flow (22%, P<0.01 and 33%, P<0.01) while 8-PT, indomethacin and 5-HD had no effect. 6. K(ATP) channels are the major mediators of the coronary reactive hyperaemic response in the guinea-pig. Although mitochondrial K(ATP) channels contribute, they appear less important than sarcolemmal channels.

Figure 1

Concentration-response curves to illustrate pinacidil (a) and diazoxide (b) responses in the presence and absence of 5-hydroxy-decanoic acid (5-HD) and glibenclamide. In the presence of 5-HD the pinacidil dose response curve was shifted to the right. Glibenclamide shifted the dose response curve further to the right. The vasodilator response to diazoxide was inhibited equally by both 5-HD and glibenclamide with a decrease in maximum vasodilator response. ^*** P<0.001 vs control for ED₅₀ and ### P<0.001 vs control for K_max.

Figure 2

Peak reactive hyperaemic flow response (a) and flow debt repayment (b) in the presence and absence of L-NAME. L-NAME had relatively little effect on the peak reactive hyperaemic flow response curves but decreased vasodilator response to short ischaemic challenges. The percentage flow debt repayment tended to be reduced in the presence of L-NAME for all ischaemic challenges, although this did not reach statistical significance. ^*P<0.05, ^**P<0.01 vs control.

Figure 3

Peak reactive hyperaemic flow response (a) and flow debt repayment (b) in the presence and absence of indomethacin. Indomethacin had very little effect on the reactive hyperaemic response such that the response to shorter ischaemic challenges is almost superimposed with a tendency for an attenuated response to longer ischaemic challenges. There were no significant differences in flow debt repayment in the presence of indomethacin.

Figure 4

Peak reactive hyperaemic flow response (a) and flow debt repayment (b) in the presence and absence of 8-PT reduced both the peak flow response and the flow debt repayment independently of the duration of the ischaemic challenge. ^*P<0.05, ^**P<0.01, ^***P<0.001 vs control.

Figure 5

Peak reactive hyperaemic flow response (a) and flow debt repayment (b) in the presence and absence of glibenclamide. Glibenclamide had a large effect on the peak hyperaemic flow response (a). Responses to ischaemic challenges of up to 40 s were reduced, with some residual vasodilator capacity to longer ischaemic challenges. Flow debt repayment was also significantly impaired by the presence of glibenclamide. ^*P<0.05, ^**P<0.01, ^***P<0.001 vs control.

Figure 6

Peak reactive hyperaemic flow response (a) and flow debt repayment (b) in the presence and absence of 5-hydroxy-decanoic acid (5-HD). 5-HD inhibited both the peak hyperaemic flow response and flow debt repayment, but to a lesser extent than glibenclamide. Despite this, 5-HD had a greater effect with longer ischaemic challenges. ^**P<0.01, ^***P<0.001 vs control.