K+-evoked depolarization induces serotonin N-acetyltransferase activity in photoreceptor-enriched retinal cell cultures. Involvement of calcium influx through l-type channels (original) (raw)

1990, Neurochemistry International

The roles of membrane depolarization and calcium influx in the regulation of retinal serotonin Nacetyltransferase (NAT) activity were investigated in low-density monolayer cultures of chick retinal cells, in which photoreceptors represented approximately 70% of the total cell population. NAT activity expressed by the cells in these cultures was markedly increased by elevating the concentration of extracellular K+. Activity increased rapidly during the first 6 h of exposure to K+, and remained elevated for at least 30 h. Chelation of calcium in the culture medium abolished the K+-evoked increase in NAT activity. Antagonists of voltage-sensitive calcium channels, nifedipine, methoxyverapamil (D600), Mr?, Me, and Cd'+ inhibited the K+-evoked increase of NAT activity. Bay K 8644, a dihydropyridine calcium channel agonist, increased NAT activity when added alone and potentiated the K+-evoked increase of activity. The effect of Bay K 8644 was antagonized by nifedipine. Addition of nifedipine 18 h after addition of K+, when NAT activity is elevated, caused activity to decrease to basal levels. These studies indicate that the increase of retinal NAT activity induced by K+-depolarization is mediated by a calcium-dependent process that involves sustained Ca2+ influx through L-type voltage-sensitive Ca'+-channels. INTRODUCI-ION Serotonin N-acetyltransferase (NAT, acetyl-CoA: arylalkylamine N-acetyltransferase, EC 2.3.1.87) is a key enzyme in the regulation of melatonin biosynthesis (Klein and Weller, 1970), which in the retina occurs primarily in photoreceptors (reviewed in Iuvone, 1986, Wiechmann, 1986; Iuvone & al, 1990). Retinal NAT activity increases at night in darkness, and is suppressed by light (Hamm and Menaker, 1980; Binkley et al., 1980; Iuvone and Besharse, 1983; Zawilska and Iuvone, 1989; Nowak et al., 1989). Studies using eyecups prepared from African clawed frogs (Xenoous laevis) suggest that the increase of retinal NAT activity that occurs at night in darkness is mediated by a calcium-dependent process (Iuvone and Besharse, 1986a).