Desensitization of cultured muscle cells to isoproterenol

Abstract

We have studied the desensitization of the muscle cell line L 6E 9 to isoproterenol as a function of cell density in both whole cells and homogenates derived from them. Undifferentiated (low density) and differentiated (high density) cells both respond to isoproterenol initially with a rapid elevation of cellular cyclic AMP. High density cells require 8 h (versus 4 h for low density cells) of continuous exposure to isoproterenol before cyclic AMP levels are reduced to control values. This difference between high and low density cells would appear to be due to greater phosphodiesterase activity in low density cells. When either low or high density cells are continuously exposed to isoproterenol for 2 h, washed and reexposed to fresh isoproterenol, cyclic AMP levels can no longer be increased above corresponding controls. In this respect, desensitization to isoproterenol in low and high density cultures proceeds with a similar time course. However, membranes prepared from intact desensitized low and high density cells show differences from each other both in isoproterenol-dependent adenylate cyclase activity and in β-adrenergic receptors. Desensitized low density cells lose, whereas desensitized high density cells retain, isoproterenol-dependent adenylate activity. On the other hand, the number of β-receptors, as detected by [ 125I]iodocyanopindolol, is unchanged in low density cells but decreased in high density cells. Furthermore, the affinity of isoproterenol for β-receptors is decreased to a greater extent in high density than in low density cells. The results indicate that desensitization of β-adrenergic catecholamines involves several different loci in the hormonal machinery of the cell and that the importance of these loci varies as a function of cellular differentiation.