Ca-permeable channels are ubiquitous and are gated by the binding of a ligand to a specific site or by a change in the membrane potential. In the present work, the cGMP gated channels of the vertebrate photoreceptors and the voltage gated Ca-channels of the hair cells of the frog semicircular canal are analyzed. The rod cGMP channel is a symmetric heterotetramer composed of two alfa and two beta subunits, all having binding sites for cGMP, (gating by cGMP <1 msec, Hll coefficient about 1.5-3, Kd about 5-50 microM). The channel is weakly voltage-dependent, and the current-voltage relations (I/V) can be fitted by the Goldman-Hodgkin-Katz equation only if a single ionic species carried the current, but not under bi-ionic conditions. The pore discriminates poorly among monovalent cations, whereas it is very permeable to Ca. However, Ca (and Mg) blocks the pore while permeating through it, thus Ca carries a smaller portion of the current with respect to Na ( about 15% vs. about 85% in physiological conditions) and reduces the channel conductance from about 25-60 pS to about 100 fS. Preliminary results indicate that the Ca permeability is modulated by the intracellular guanosine level. The Ca-channels of the hair cells of the semicircular canals are poorly known from an electrophysiological and a molecular point of view. As many other Ca-channels, the largest current (up to 300 pA) is attained at -20 mV, the reversal potential is smaller (about +20 mV) than the one predicted by the Nernst equation, and the current inactivates to some extent during depolarizing steps (10-280 ms duration). With time, the current becomes progressively smaller and the gating kinetics progressively slower (run-down), up to a point that no more functional channels are present. Previous data suggest that a single L-type Ca-channel exists in the hair celi membrane. This view is challenged by the following evidences: only in few of the cells Ca-free, 4 mM Ba solutions produce a two fold increase in the inward current, and 5 pM Bay K 8644, applied either extra- or intracellularly, increases the Ca-current with small changes in the I-V curve.
GATING AND SELECTIVITY OF CA-PERMEABLE CHANNELS OF SENSORY CELLS
RISPOLI, Giorgio;MARTINI, Marta;RUBBINI, Gemma;ROSSI, Marialisa
1999
Abstract
Ca-permeable channels are ubiquitous and are gated by the binding of a ligand to a specific site or by a change in the membrane potential. In the present work, the cGMP gated channels of the vertebrate photoreceptors and the voltage gated Ca-channels of the hair cells of the frog semicircular canal are analyzed. The rod cGMP channel is a symmetric heterotetramer composed of two alfa and two beta subunits, all having binding sites for cGMP, (gating by cGMP <1 msec, Hll coefficient about 1.5-3, Kd about 5-50 microM). The channel is weakly voltage-dependent, and the current-voltage relations (I/V) can be fitted by the Goldman-Hodgkin-Katz equation only if a single ionic species carried the current, but not under bi-ionic conditions. The pore discriminates poorly among monovalent cations, whereas it is very permeable to Ca. However, Ca (and Mg) blocks the pore while permeating through it, thus Ca carries a smaller portion of the current with respect to Na ( about 15% vs. about 85% in physiological conditions) and reduces the channel conductance from about 25-60 pS to about 100 fS. Preliminary results indicate that the Ca permeability is modulated by the intracellular guanosine level. The Ca-channels of the hair cells of the semicircular canals are poorly known from an electrophysiological and a molecular point of view. As many other Ca-channels, the largest current (up to 300 pA) is attained at -20 mV, the reversal potential is smaller (about +20 mV) than the one predicted by the Nernst equation, and the current inactivates to some extent during depolarizing steps (10-280 ms duration). With time, the current becomes progressively smaller and the gating kinetics progressively slower (run-down), up to a point that no more functional channels are present. Previous data suggest that a single L-type Ca-channel exists in the hair celi membrane. This view is challenged by the following evidences: only in few of the cells Ca-free, 4 mM Ba solutions produce a two fold increase in the inward current, and 5 pM Bay K 8644, applied either extra- or intracellularly, increases the Ca-current with small changes in the I-V curve.I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
