Single calcium channels in resistance-sized cerebral arteries from rats

Unitary currents through single calcium channels were measured from cell-attached patches on smooth muscle cells isolated from resistance-sized branches of posterior cerebral arteries from Wistar-Kyoto normotensive rats. Barium (80 and 10 mM) was used as the charge carrier, with and without the dihydropyridine calcium channel agonist BAY R 5417. Unitary currents decreased on membrane depolarization, with a slope conductance of 19.4 pS (80 mM barium). Channel open-state probability (Po) was steeply voltage dependent. Peak Po during test pulses from -70 mV increased e-fold per 4.5-mV depolarization. Mean peak Po at potentials positive to +10 mV was 0.44. Po at steady membrane potentials was also steeply voltage dependent, changing e-fold per 4.5 mV in the absence of inactivation. Steady-state Po at positive potentials was substantially lower than peak Po elicited by test pulses, suggesting that steady-state inactivation can reduce Po by as much as 10-fold. Membrane depolarization decreased the longest mean closed time but had little effect on the mean open time of single calcium channels measured during steady-state recordings. Lowering the external barium concentration from 80 to 10 mM reduced the single channel conductance to 12.4 pS and shifted the relationship between steady-state Po and membrane potential by about -30 mV. BAY R 5417 also shifted this relationship by about -15 mV.

The calyx-type synapse of the chick ciliary ganglion as a model of fast cholinergic transmission

The calyx-type synapse of the embryonic chick ciliary ganglion is reviewed as a model of transmitter release from a vertebrate presynaptic nerve terminal. This nerve terminal is extensive in area, enabling the penetration with microelectrodes and the application of patch-clamp techniques. In other respects the calyx synapse is a typical fast-transmitting cholinergic nerve terminal. This synapse has been used to obtain the first recordings of action potentials and calcium currents in a vertebrate presynaptic nerve terminal and is the only preparation in which it has proved possible to record single calcium channels directly from the transmitter release sites. The calyx remains a powerful experimental preparation for the further analysis of the mechanism and control of neurotransmitter release in fast-transmitting nerve terminals.Key words: synaptic transmission, presynaptic, transmitter release, acetylcholine release, calcium channels.

Regulation of single calcium channels in cerebral arteries by voltage, serotonin, and dihydropyridines

Unitary currents through Ca channels were measured from cell-attached patches on smooth muscle cells isolated from rabbit cerebral (basilar) arteries. Barium (80 and 10 mM) and calcium (80 and 10 mM) were used as the charge carriers. The dihydropyridine Ca channel agonist BAY R 5417 was used to increase open-state probability (Popen), with 500 nM BAY R 5417 increasing Popen 10-fold at 0 mV. Barium currents through single Ca channels were greater than calcium currents at any voltage, with single-channel conductances negative to -20 mV of 24.6 pS (80 mM barium), 15.1 pS (80 mM calcium), 17.2 pS (10 mM barium), and 5.8 pS (10 mM calcium). The single-channel Popen increased 2.7-fold per 5- to 7-mV membrane depolarization (negative to 0 mV) and was half maximal at +0.4 mV (80 mM calcium) and +13.5 mV (80 mM barium). Ca channels with calcium but not with barium as the charge carrier exhibited pronounced inactivation positive to -20 mV (half time, 112 ms at 0 mV). The dihydropyridine nimodipine (2 nM) inhibited average currents through Ca channels. The cerebral artery constrictor serotonin increased Popen of single Ca channels by as much as 200-fold without an effect on single-channel conductance. A second distinct amplitude of unitary currents was often observed, corresponding to a channel conductance of about one-half the more commonly observed level. The small-conductance-level channel was voltage dependent, did not inactivate over 0.5-s test pulses (with barium), and could be activated by serotonin.