Rapid electrical stimulation of contraction reduces the density of beta-adrenergic receptors and responsiveness of cultured neonatal rat cardiomyocytes - Possible involvement of microtubule disassembly secondary to mechanical stress

Background-Although tachycardia is commonly present in patients with conges tive heart failure, its role in the development of congestive heart failure remains unclear. We studied the effect of rapid electrical stimulation of contraction on beta-adrenergic receptor (beta-AR) signal pathway in culture d cardiomyocytes of neonatal rats. Methods and Results-Contraction of cardiomyocytes was induced by electrical stimulation at 50 V with twice the threshold pulse width, beta-ARs were id entified by [H-3]CGP-12177 and [H-3]dihydroalprenolol. Electrical stimulati on reduced cell-surface but not total beta-AR density; the effect was depen dent on pacing frequency (a reduction of 11%, 28%, and 18% in cells paced a t 2.5, 3.0, and 3.3 Hz, respectively). This reduction was apparent at 3 hou rs, in contrast to reduced beta-AR density after exposure to isoproterenol (ISP) for 1 hour. The fraction and inhibition constant of beta-AR binding a gonist with high affinity were not affected by rapid electrical stimulation . In cardiomyocytes paced at 3.0 Hz for 24 hours, the response to ISP decre ased compared with unpaced cells, 142% versus 204% of baseline with 1 mu mo l/L ISP, whereas the responses to forskolin or acetylcholine were not diffe rent. Treatment of cardiomyocytes with 2,3-butanedione monoxime (10 mmol/L) or taxol (10 mu mol/L) inhibited the rapid pacing-induced reduction in bet a-AR density. Conclusions-Our results suggest that contractile activity is involved in re gulation of cardiac function by modulating the beta-AR system independently of hemodynamic and neurohormonal factors. This may help to elucidate the r ole of mechanical stress in the development of heart failure.