The effect of pertussis toxin on β‐adrenoceptor responses in isolated cardiac myocytes from noradrenaline‐treated guinea‐pigs and patients with cardiac failure

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Abstract
1 A decreased responsiveness to the positive inotropic effects of β-adrenoceptor agonists is a characteristic of human heart failure. We have investigated the involvement of inhibitory guanine nucleotide binding proteins (G-proteins) in this process using pertussis toxin treatment of isolated cardiac myocytes. 2 Myocytes isolated from failing human myocardium had a reduced maximum contractile response to isoprenaline relative to that for maximally stimulating concentrations of calcium, giving an isoprenaline/calcium ratio of 0.71 ± 0.06 (n = 7 patients). This was significantly lower than in myocytes from non-failing myocardium, where the isoprenaline/calcium ratio was 1.16 ± 0.07 (n = 6, P < 0.001). Responses to high calcium were unchanged. 3 Myocytes were treated with pertussis toxin for 3–5 h at 35°C. Successful inactivation of inhibitory G-proteins by pertussis toxin treatment was indicated by a loss of responsiveness to 10 μm adenosine (in the presence of submaximal isoprenaline). 4 Following pertussis toxin treatment of the myocytes from failing human heart the isoprenaline/calcium ratio increased to 1.43 ± 0.27 (n = 7, P < 0.05). Pertussis toxin treatment had no effect upon the maximum calcium contraction. The isoprenaline/calcium ratio in myocytes from non-failing human ventricle was not affected by the toxin treatment (n = 3). These observations support the hypothesis that increased inhibitory G-protein levels or activities contribute to β-adrenoceptor desensitization in human heart failure. 5 β-Adrenoceptor desensitization in human heart failure is thought to be secondary to raised noradrenaline levels in these patients. Experiments were repeated on myocytes isolated from hearts of guinea-pigs which had been chronically infused with noradrenaline. The isoprenaline/calcium ratio of these myocytes was reduced below that of myocytes from sham-operated animals (0.65 ± 0.04, n = 6 compared with 0.88 ± 0.02, n = 7, P < 0.001). 6 Pertussis toxin treatment (2 h at 35°C) increased the isoprenaline/calcium ratio to 1.02 ± 0.02 (n ≅ 6, P < 0.01) in myocytes from noradrenaline-treated guinea-pigs. This effect of pertussis toxin treatment was not seen in myocytes from sham-operated guinea-pig hearts. 7 Incubation at 35°C for similar periods in the absence of pertussis toxin also restored the isoprenaline/calcium ratio towards normal in the myocytes from both failing human and noradrenaline-treated guinea-pig hearts, although the effect was significantly smaller than that produced by the toxin. Myocytes kept at room temperature (22°C) showed no such evidence of resensitization over periods up to 6 h. 8 These observations are consistent with the hypothesis that raised catecholamine levels result in increased inhibitory G-protein levels and functional β-adrenoceptor desensitization in heart failure.

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