CONTINUOUS MONITORING OF RECEPTOR-MEDIATED CHANGES IN THE METABOLIC RATES OF LIVING CELLS

Activation of β-adrenergic or muscarinic acetylcholine receptors expressed in transfected cells or epidermal growth factor receptors in human keratinocytes produces 15% to 200% changes in cellular metabolic rates. Changes in cell metabolism were monitored continuously with a previously described silicon-based microphysiometer that detects small changes in extracellular pH. The amplitude and kinetics of the metabolic changes depend upon several factors including pretreatment of the cells prior to receptor stimulation, the dose of hormone/neurotransmitter used, and the receptor complement of the cells. Responses are receptor specific; cells transfected with receptor genes respond only to the appropriate hormone/transmitter, whereas control (nontransfected) cells or cells transfected with different receptors exhibit no response. The specificity of the responses was further documented by using pharmacological antagonists. In Chinese hamster ovary (CHO) cells transfected with human β2-adrenergic receptors, isoproterenol produces a 20-60% increase in the rate of extracellular acidification with an EC50 of 4 nM, a response that is competitively antagonized by (-)-propranolol. The EC50 for the isoproterenol response is shifted from 4 nM to 100 nM in the presence of 3 nM (-)-propranolol. The kinetics of the metabolic response induced by β-adrenergic receptor stimulation are markedly slower than those elicited by muscarinic receptor agonists. The maximal metabolic response in cells transfected with β-adrenergic receptors peaks at ≈12 min as compared with β30 sec in cells transfected with muscarinic receptors, perhaps reflecting activation of different second-messenger pathways. These findings illustrate an alternative means of studying cellular responses to hormones and neurotransmitters and suggest that metabolic changes will be generally useful for detecting the consequences of receptor-ligand interactions.