Block of the L-type Ca2+ channel pore by external and internal Mg2+ in rat phaeochromocytoma cells.

摘要: 1. Three to eight micromolar external Mg2+ produces discrete block of the unitary inward currents through L-type Ca2+ channel carried by 300 mM Li+. Like block, increasing Li+ concentration decreases on-rate and increases off-rate. 2. These kinetic changes are saturating apparent dissociation constant (Kd) for on-rates in 75 (in activity), same as that case block. This suggests produce at site. The Kd off-rates is mM, much smaller than indicates exerts less repulsion on ion neighbouring (enhancement) site Ca2+, although both have two charges. theoretical fits also suggest can exit blocking sites a rate several hundred per second absence any enhancement effect. 3. Seventeen forty-eight internal outward general approximately 20 times faster compared currents. finding high-affinity more easily outside inside, implying significantly higher energy barriers inner side Mg2+. 4. At least 5-10 needed 215 Na+. such experiments generally those reach site, namely binding pore, 5. Other size current. most probably due fast internally located low-affinity pore. fractional decrease voltage dependent be fitted rectangular hyperbola calculate Kd, which e-fold 45 mV hyperpolarization, indicating an electrical distance 0.3 between pore mouth.(ABSTRACT TRUNCATED AT 400 WORDS)