1. High-voltage-activated calcium currents were studied with the whole-cell, patch-clamp technique in acutely dissociated pyramidal neurones from guinea-pig piriform cortex layer II. Barium ions were used as charge carriers. 2. Barium currents (I(Ba)) displayed a remarkable kinetic diversity in different neurones. The ratio between the current amplitude at the end of the test pulses and the peak amplitude (R(e/p)) showed two frequency-distribution peaks at approximately 0.4 and 0.8. The index of current activation speed (rise time 10-90%) directly correlated with the index of current persistence, R(e/p). 3. The half-activation potential (V( 1/2 )) of total I(Ba)s positively correlated with the R(e/p) of the corresponding currents. This implied that the high-decay I(Ba)s also had a more negative voltage range of activation than the more persistent ones. 4. The L- and N-type channel blockers nifedipine (10 μM) and ω-conotxin in GVIA (ω-CTx GVIA, 0.5-1 μM) additively blocked 20 and 25% of the total I(Ba), respectively. The P/Q-type calcium channel blockers ω-agatoxin IVA (100 nM), ω-conotoxin MVIIC (1 μM) and 3.3 funnel toxin (1 μM), had little effect on I(Ba). 5. The nifedipine- and ω-CTx GVIA-sensitive current had a R(e/p) > 0.55 and their voltage dependence of activation was of the high voltage-activated type (V( 1/2 ) ≃ 0 mV). 6. High-, intermediate- and low-decay blocker-resistant currents were observed in different neurones. Their R(e/p) values highly correlated with those of the corresponding total I(Ba)s and with the voltage dependence of activation of the underlying conductances. Exponential fittings of the inactivation phase of blocker-resistant currents returned very fast time constants (lower than 30 ms) for high-decay currents (R(e/p) <0.25). The intermediate-decay currents (R(e/p) ≃ 0.55) could not derive from variable combinations of high- and low-decay current components. 7. Our data demonstrate a remarkable variety in voltage-activated calcium currents expressed by piriform cortex neurones, that include currents resistant to high-voltage-activated calcium-channel blockers.
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