Evoked phasic release in frog nerve terminals obtained after block of Ca2+entry by Cd2+

✍ Scribed by J. Dudel; H. Parnas; I. Parnas

• Publisher: Springer
• Year: 1991
• Tongue: English
• Weight: 777 KB
• Volume: 419
• Category: Article
• ISSN: 0031-6768
• DOI: 10.1007/bf00373007

✦ Synopsis

Cutaneous pectoris muscles of frogs were isolated, mounted in a chamber and superfused with Ringer's solution. With a macro-patch-clamp electrode [5] placed on a section of a motor nerve terminal, quantal synaptic currents were elicited by depolarizing pulses and recorded. The electorde tip and the section of the terminal recorded from were perfused rapidly by Ringer's solution alone or containing 20-500 p~M Cd 2+ to block Ca 2+ inflow. Separate superfusion of the muscle and the rest of the terminal with normal or elevated Ca 2+ Ringer's solution provided a sufficiently high resting Ca 2+ concentration in the terminal even when Ca 2+ inflow was blocked by Cd 2+ . The depolarization level of maximal Ca z+ inflow into the terminal was found by measuring maximal test pulse facilitation,/~ [6]. In control solution as well as in the case of Cd 2+ block, the rate of phasic release after depolarizing pulses rose further when depolarization was increased past the level of/~c, and reached a saturation level which was maintained at estimated depolarizations up to +200mV. Block of Ca 2+ inflow by Cd 2+ decreased release substantially, but did not suppress it. The depression of release was greater in the range of large Ca 2+ inflow (around Fc) than for very large depolarizations. The time course of phasic release was unaltered by blockage of CaZ+inflow. It is concluded that Ca 2+ inflow contributes to the promotion of evoked release only in the depolarization range in which Ca 2+ inward current is large. When Ca 2+ concentration in the terminal is sufficiently high, release can be evoked by depolarization in the absence of Ca 2+ inflow, the voltage-dependent release factor, S, compensating to a great extent for the lack of Ca 2+ inflow.