Potential Novel Mechanism of Porcine Coronary Artery Hypoxic Vasorelaxation Involving Hydrogen Sulphide and Adenosine

Michael Garle, Stephen Alexander, William Dunn, Vera Ralevic. School of Biomedical Sciences, University of Nottingham, NG7 2UH, Nottingham, UK.

Exposure to hypoxia causes the coronary vasculature to dilate, but the mechanism is not fully defined. There is a body of evidence indicating that adenosine is involved in hypoxic vasodilatation and, more recently, hydrogen sulphide (H2S) has been reported to act as an oxygen sensor/transducer of hypoxic responses, in rat and lamprey aorta, and rat and bovine pulmonary arteries (Olson et al., 2006). H2S is synthesised in biological tissues mainly by the actions of the enzymes cystathionine γ-lyase (CSE/CGL) and cystathionine β-synthase (CBS). In the present study we investigated the roles of H2S and adenosine in hypoxic vasorelaxation of the porcine coronary artery.

Pig hearts were obtained on ice from a local abattoir. Segments of coronary arteries were mounted for isometric tension recording in warmed, oxygenated, Krebs-Henseleit buffer (Rayment et al., 2007). Tissue viability was assessed by eliciting contractions with 60 mM KCl. Arteries were preconstricted with U46619, a thromboxane A2 mimetic. Hypoxia was induced by gassing with 95% N2, 5% CO2. Hypoxic responses were investigated in the absence and presence of inhibitors of CSE and CBS. In separate experiments the effects of H2S, added as the donor Na2S, were investigated on relaxations to adenosine, NECA and isoprenaline. Contractions were expressed as a percentage of the response to KCl, and relaxations as a percentage of U46619-induced tone. Data were analysed using two-way ANOVA or Student’s t test. P<0.05 was taken as significant.

Porcine coronary arteries elicited a biphasic response upon exposure to hypoxia, with a transient contraction followed by a prolonged vasorelaxation; both responses were attenuated by inhibition of H2S synthesis using D,L-propargylglycine (PPG, 10 µM) and amino-oxyacetate (AOAA, 100 µM): contraction control 13 ± 1%, plus PPG and AOAA 6 ± 2% (P<0.01, n = 6); relaxation control 87 ± 4%, plus PPG and AOAA 62 ± 6% (P<0.001, n = 7). PPG and AOAA had no significant effect on relaxations to pinacidil (0.01-10 µM, n = 5-7). Exogenous H2S elicited vasorelaxation at concentrations greater than 30 µM. H2S, at a concentration (10 µM) subthreshold for direct vasorelaxation, augmented relaxations to adenosine; for example, adenosine (3 µM) control 8 ± 2% (n = 6), presence of H2S 36 ± 8% (n = 8) (P<0.05). Relaxations to NECA (10 nM) were also augmented by H2S (10 µM); NECA control, 20 ± 4% (n = 11), presence of H2S, 38 ± 6% (n = 13) (P<0.05). Amplitude-matched relaxations to isoprenaline were not significantly different in the absence and presence of H2S (10 µM): isoprenaline (10 nM) control, 4 ± 1% (n = 6), with H2S, 4 ± 1% (n = 7); isoprenaline (30 nM) control 11 ± 2% (n = 6), with H2S 12 ± 2% (n = 7).

These data indicate that endogenous H2S is a mediator of hypoxic vasodilatation of the porcine coronary artery; in addition, H2S can selectively augment vasorelaxation to adenosine, a known cardioprotective agent. These data identify a novel mechanism of hypoxic vasodilatation in which H2S can both cause direct vasorelaxation, and selectively augment adenosine-induced vasorelaxation; in this dual effect, sensitization of adenosine relaxation appears to occur at a lower concentration of H2S than required for direct vasorelaxation.

Olson, KR, et al. (2006). J Exp Biol 209: 4011-4023.

Rayment, SJ, et al. (2007). FASEB J 21(2): 577-585.