The Journal of Neurophysiology Vol. 88 No. 2 August 2002, pp. 879-887
Copyright ©2002 by the American Physiological Society

Extrusion of Intracellular Calcium Ion After In Vitro Ischemia in the Rat Hippocampal CA1 Region

 ¹Department of Physiology and  ²Department of Neurosurgery, Kurume University School of Medicine, 67 Asahi-machi, Kurume 830-0011, Japan

Tanaka, E., H. Uchikado, S. Niiyama, K. Uematsu, and H. Higashi. Extrusion of Intracellular Calcium Ion After In Vitro Ischemia in the Rat Hippocampal CA1 Region. J. Neurophysiol. 88: 879-887, 2002. Simultaneous recordings of intracellular Ca²⁺ ([Ca²⁺]_i) signal and extracellular DC potential were obtained from the CA1 region in 1-[6-amino-2-(5-carboxy-2-oxazolyl)-5-benzofuranyloxy]-2-(2-amino-5-methylphenoxy)-ethane-N,N,N',N'-tetraacetic acid penta-acetoxymethyl ester (Fura-2/AM)-loaded rat hippocampal slices. Superfusion with oxygen- and glucose-deprived medium (in vitro ischemia) for 5-6 min produced a rapid rise of the [Ca²⁺]_i level in the stratum radiatum (rising phase of the [Ca²⁺]_i signal), which occurred simultaneously with a rapid negative DC potential (rapid negative potential). When oxygen and glucose were reintroduced, the increased [Ca²⁺]_i signal diminished rapidly (falling phase of the [Ca²⁺]_i signal) during the generation of a slow negative DC potential (slow negative potential), which occurred within 1 min from the onset of the reintroduction. Thereafter, the [Ca²⁺]_i signal partially and the slow negative potential completely returned to the preexposure level approximately 6 min after the reintroduction. The changes in [Ca²⁺]_i signal during and after in vitro ischemia were very similar to the changes in the membrane potential of glial cells. The rising and falling phases of [Ca²⁺]_i signal corresponded to the rapid depolarization and a depolarizing hump, respectively, in the repolarizing phase of glial cells. A prolonged application of in vitro ischemia or a reintroduction of either glucose or oxygen suppressed the falling phase after ischemic exposure. The application of ouabain (30 µM) generated both a rapid negative potential and a rapid elevation of [Ca²⁺]_i, but no slow negative potential or rapid reduction in [Ca²⁺]_i were observed. When oxygen and glucose were reintroduced to slices in the Na⁺-free or ouabain- or Ni²⁺-containing medium, the falling phase was suppressed. The falling phase was significantly accelerated in Ca²⁺- and Mg²⁺-free with EGTA-containing medium. In contrast, the falling phase was significantly slower in the Ca²⁺-free with high Mg²⁺- and EGTA-containing medium. The falling phase of the [Ca²⁺]_i signal after ischemic exposure is thus considered to be primarily dependent on the reactivation of Na⁺, K⁺-ATPases, while the extrusion of cytosolic Ca²⁺ via the forward-mode operation of Na⁺/Ca²⁺ exchangers in glial cells is thought to be directly involved in the rapid reduction of [Ca²⁺]_i after ischemic exposure.