ENDOGENOUS DHP-SENSITIVE CA2+ CHANNELS IN PLEURODELES OOCYTES

The double electrode voltage-clamp technique was used to study voltage -dependent Ca2+ channels in Pleurodeles oocytes. From a holding potent ial of -80 mV, Ba-current (I-Ba) (recorded in Cl-free solution, Ba2+ = 40 mM) activated at -36.7 +/- 4 mV, peaked at -11.6 +/- 4 mV and reve rsed at 55 +/- 7 mV (n = 24). This current activated slowly (rise time was 0.98 +/- 0.2 s; n = 14 at -10 mV) and was not inactivated. Cadmiu m (Cd2+, 500 mu M) completely inhibited I-Ba. The effect of Cd2+ was d ose-dependent(EC(50) = 37 +/- 5 mu M; n = 5). Moreover, I-Ba was insen sitive to omega-conotoxin (10 mu M) but interestingly this I-Ba displa yed dihydropyridine (DHP) sensitivity. Bay K 8644 (5 mu M), a DHP acti vator, increased the peak current amplitude in a dose-dependent manner (EC(50) = 5.9 +/- 0.6 mu M; n = 10) and shifted the threshold and the maximum of current/voltage relationship towards negative potentials b y -10 mV. Nifedipine (5 mu M), a DHP antagonist, decreased I-Ba by 80% at HP of -80 mV (EC(50) = 1.2 +/- 0.2 mu M; n = 6). We concluded that Pleurodeles oocytes possess High-Voltage Activated Ca2+ channels with properties similar to L-type Ca2+ channels.