CORONARY-ARTERY CONSTRICTION IN RATS AFFECTS THE ACTIVATION OF ALPHA(1) ADRENERGIC-RECEPTORS IN CARDIAC MYOCYTES

Objective: To determine whether alpha(1) adrenergic receptor mediated myocyte contractility and growth are depressed acutely after non-occlu sive coronary artery narrowing, the left coronary artery was constrict ed in rats and mechanical behaviour, cytosolic calcium, and regulation of alpha(1) adrenergic receptors were examined in myocytes seven days later, Methods: Coronary artery stenosis was surgically induced in ra ts and following the estimation of global cardiac performance myocytes were enzymatically dissociated and radioligand binding studies were p erformed. In addition, the isotonic contractile performance, cytosolic calcium transients and noradrenaline stimulated inositol phosphate ge neration in myocytes were measured in the presence of WB 4101 or after chlorethylclonidine treatment. Results: Estimations of cell mechanics in vitro established that peak shortening was decreased by 36% and 18 % in left and right ventricular myocytes of coronary stenosed rats. Ti me to peak shortening was prolonged by 29% in left and 20% in right my ocytes, whereas velocity of shortening was decreased by 27% in left my ocytes. These alterations were associated with increases in cell lengt h and width, indicative of myocyte hypertrophy. In addition, coronary stenosis was accompanied by reductions in the expression of alpha(1a) and alpha(1b) receptor subtypes in myocytes. alpha(1) Adrenergic recep tor density and noradrenaline stimulated phosphoinositol turnover were decreased by 30% and 34% in left myocytes. alpha(1a) Adrenergic recep tor subtype mediated cytosolic calcium concentration and myocyte mecha nical performance were also impaired in left myocytes only. The alpha( 1a) adrenergic receptor subtype antagonist WB 4101 abolished noradrena line stimulated inositol phosphate generation in myocytes, whereas chl orethylclonidine at large doses only partially inhibited this response . Conclusions: In conclusion, coronary narrowing leads to defects in t he regulation of alpha(1) adrenergic receptors on myocytes which are c oupled with attenuation in the transmission of signals, possibly affec ting myocyte cell function and ongoing reactive cellular hypertrophy.