The expression of plasma membrane Ca2+ pump isoforms in cerebellar granuleneurons is modulated by Ca2+

Plasma membrane Ca2+ ATPase (PMCA) pump isoforms 2, 3, and 1CII are express ed in large amounts in the cerebellum of adult rats but only minimally in n eonatal cerebellum. These isoforms were almost undetectable in rat neonatal cerebellar granule cells 1-3 days after plating, but they became highly ex pressed after 7-9 days of culturing under membrane depolarizing conditions (25 mM KCl). The behavior of isoform 4 was different: it was clearly detect able in adult cerebellum but was down-regulated by the depolarizing conditi ons in cult-red cells. 25 mM KCl-activated L-type Ca2+ channels, significan tly increasing cytosolic Ca2+. Changes in the concentration of Ca2+ in the culturing medium affected the expression of the pumps. L-type Ca2+ channel blockers abolished both the up-regulation of the PMCA1CII, 2, and 3 isoform s and the down-regulation of PMCA4 isoform, When granule cells were culture d in high concentrations of N-methyl-D-aspartic acid, a condition that incr eased cytosolic Ca2+ through the activation of glutamate operated Ca2+ chan nels, up regulation of PMCA1CII, 2, and 3 and down-regulation of PMCA4 was also observed. The activity of the isoforms was estimated by measuring the phosphoenzyme inter mediate of their reaction cycle: the up-regulated isofo rms, the activity of which was barely detectable at plating time, accounted far a large portion of the total PMCA activity of the cells. No up-regulat ion of the sarcoplasmic/endoplasmic reticulum calcium pump was induced by t he depolarizing conditions.