title: Calcium entry in squid axons during voltage clamp pulses.
abstract: Squid giant axons were injected with aequorin and tetraethylammonium and were impaled with sodium ion sensitive, current and voltage electrodes. The axons were usually bathed in a solution of varying Ca2+ concentration ([Ca2+]o) containing 150mM each of Na+, K+ and an inert cation such as Li+, Tris or N-methylglucamine and had ionic currents pharmacologically blocked. Voltage clamp pulses were repeatedly delivered to the extent necessary to induce a change in the aequorin light emission, a measure of axoplasmic Ca2+ level, [Ca2+]i. The effect of membrane voltage on [Ca2+]i was found to depend on the concentration of internal Na+ ([Na+]i). Voltage clamp hyperpolarizing pulses were found to cause a reduction of [Ca2+]i. For depolarizing pulses a relationship between [Ca2+]i gain and [Na+]i indicates that Ca2+ entry is sigmoid with a half maximal response at 22 mM Na+. This Ca2+ entry is a steep function of [Na+]i suggesting that 4 Na+ ions are required to promote the influx of 1 Ca2+. There was little change in Ca2+ entry with depolarizing pulses when [Ca2+]o is varied from 1 to 10mM, while at 50mM [Ca2+]o calcium entry clearly increases suggesting an alternate pathway from that of Na+/Ca2+ exchange. This entry of Ca2+ at high [Ca2+]o, however, was not blocked by Cs+o. The results obtained lend further support to the notion that Na+/Ca2+ exchange in squid giant axon is sensitive to membrane voltage no matter whether this is applied as a constant change in membrane potential or as an intermittent one.
authors: Centro de Biociencias, Instituto Internacional de Estudios Ananzados (IDEA), Caracas, Venezuela.
authors: J
authors: J
authors: Requena
authors: J
authors: J
authors: Whittembury
authors: L J
authors: LJ
authors: Mullins
mesh_terms: D000331:Aequorin / Q000494:pharmacology; D000818:Animals; D001369:Axons / Q000187:drug effects / Q000378:metabolism*; D002118:Calcium / Q000378:metabolism*; D002352:Carrier Proteins / Q000378:metabolism; D049832:Decapodiformes; D007473:Ion Channels / Q000378:metabolism*; D007700:Kinetics; D008564:Membrane Potentials; D011188:Potassium / Q000378:metabolism; D012964:Sodium / Q000378:metabolism; D019831:Sodium-Calcium Exchanger; D019789:Tetraethylammonium; D013757:Tetraethylammonium Compounds / Q000494:pharmacology
chemical_list: D002352:Carrier Proteins; D007473:Ion Channels; D019831:Sodium-Calcium Exchanger; D013757:Tetraethylammonium Compounds; D000331:Aequorin; D019789:Tetraethylammonium; D012964:Sodium; D011188:Potassium; D002118:Calcium