L-type Ca²⁺ channels in fetal and adult ovine cerebral arteries

Recently, we reported that, whereas in cerebral arteries of the adult a majority of norepinephrine (NE)-induced increase in intracellular Ca²⁺ concentration ([Ca²⁺]_i) comes from release of the sarcoplasmic reticulum (SR) Ca²⁺ stores, in the fetus the SR Ca²⁺ stores are relatively small, and NE-induced increase in [Ca²⁺]_i results mainly from activation of plasma membrane L-type Ca²⁺ channels (20). In an effort to establish further the role of L-type Ca²⁺ channels in the developing cerebral arteries, we tested the hypothesis that, in the fetus, increased reliance on plasmalemmal L-type Ca²⁺ channels is mediated, in part, by increased L-type Ca²⁺ channel density. We used ³H-labeled (+)isopropyl-4-(2,1,3-benzoxadiazol-4-y1)-1,4-dihydro-(2,6- dimethyl-5-methoxycarbonyl)pyridine-3-carboxylate (PN200-110, isradipine) to measure L-type Ca²⁺ channel density (B_max) in the cerebral arteries, common carotid artery (CCA), and descending aortae of fetal (∼140 gestation days), newborn (7-10 days), and adult sheep. In the cerebral and common carotid arteries, B_max values (fmol/mg protein) of fetuses and newborns were significantly greater than those of adults. Western immunoblotting assay also revealed that the density of L-type Ca²⁺ channel protein in the cerebral arteries and CCA was about twofold greater in the fetus than the adult. Finally, compared with the adult, fetal cerebral arteries demonstrated a significantly greater maximum tension and [Ca²⁺]_i in response to stimulation with the L-type Ca²⁺ channel agonist Bay K 8644. In addition, Bay K 8644-stimulated fetal vessels demonstrated a maximal tension and [Ca²⁺]_i similar to that observed in response to stimulation with 10^-4 NE. These results support the idea that fetal cerebrovascular smooth muscle relies more on extracellular Ca²⁺ and L-type Ca²⁺ channels for contraction than does the adult and that this increased reliance is mediated, in part, by greater L-type Ca²⁺ channel density. This may have important implications in the regulation of cerebral blood flow in the developing organism.