Ca2 + Dynamics in chemoreceptor cells: an overview

21 Ca2+ DYNAMICS IN CHEMORECEPTOR CELLS: AN OVERVIEW C. González,J.R. López-López,A. Obeso,A. Rachee and J. García -Sancho Opto. Bioquímica y Biología Molecular y Fisiología, Facultad de Medicina, Universidad de Valladolid, 47005 Valladolid, Spain The carotid body (CB) was defined as a sensory organ by De Castro in 1928. Two years later, Heymanns and coworkers demostrated that the organ was sensitive to alterations in blood gases and pH, in such a way that a decrease in blood P02 or pH or an increase in blood PC02 produced activation of the CB and, reflexely, hyperventilation. De Castro postulated that glomus cells were the sensor structures and that they should release sorne substance to transmit the stimulus to the sensory nerve endings (De Castro, 1928). De Castro's point of view, was widely accepted, and therefore the CB was considered a secondary sensory receptor. As a consequence, the principal aims of man y workers in the chemoreception field have been to define the nature of the sensing mechanims ( sensory transduction process ) and to identify the substances released by chernoreceptor cells. The initial experimental approaches to these questions were based on the use of "in vivo" preparations. The physiological role of CB in systemic reflexes was established and the "in vivo" preparation led also to sorne hypothesis about the sensory transduction process. The metabolic and the acidic hypothesis were developed (Fidone and Gonzalez 1986). However, the truly intimate functional aspects of the chemoreception process remained undefined. It became necessary to look for alternative experimental approaches, and an elegant "in vitro" preparation was developed by Eyzaguirre and Lewin in 1961. However Eyzaguirre's preparation was not well accepted; the results obtained in "in vitro" preparations were criticized, arguing that the CB was not functional or that it was dead (see diseussion in Torrance, pp. 248 and 298 and in Acker et al., 1977 pp. 76-77). General acceptance of the preparation had to await until the seventies, when Whalen and Nair (1976) and Starlinger and Lübbers (1976) demostrated that the CB "in vitro" exhibited P02 levels and 02 consumption rates very similar those found "in vivo". Ultrast.rnctural analysis of the CB after"in vitro" superfusion showedexcellentpreservation of the tissue (Verna et al., 1981). The "in vitro" preparation offered a well controlled system to study basic mechanisms, including neurotransmitter synthesis and release. In the la ter sixties, a considerable background existed in the field of neurotransmission and secretion in general. Katz and Miledi ( 1965) had demostrated the role of Ca2+ in the release of acetylcholine in the neuromuscular junction. Studies perfo1med in the adrenal medulla led to similar results and to the proposal of the concept of stimulus-secretion coupling by Douglas in 1968. In this scheme, acetylcholine would produce an increase in membrane permeability, an in flux of Ca2+ and the secretion of catecholamines. Douglas viewed stimulus-secretion eoupling as a general concept, that should be common to man y different cells; the differences would be the nature of the stimulus and of the released substances. Secretion studies caiTied out in other preparations as neurohypophysis Neurobiology and Ce/1 Physiology of Chenwreception Edited by P.G. Data et al., Plenum Press, New York, 1993 149