CALCIUM-CHANNEL CURRENTS IN ACUTELY DISSOCIATED INTRACARDIAC NEURONS FROM ADULT-RATS

With the use of the whole cell patch-clamp technique, multiple subtype s of voltage-activated calcium channels, as indicated by measuring Ba2 + currents, were pharmacologically identified in acutely dissociated i ntracardiac neurons from adult rats. All tested neurons that were held at -80 mV displayed only high-voltage-activated (HVA) Ca2+ channel cu rrents that were completely blocked by 100 mu M CdCl2. The current den sity of HVA Ca2+ currents was dependent on the external Ca2+ concentra tion. The Ba2+ (5 mM) currents were half-activated at -16.3 mV with a slope of 5.6 mV per e-fold change. The steady-state inactivation was a lso voltage dependent with half-inactivation at -33.7 mV and a slope o f -12.1 mV per e-fold change. The most effective L-type channel activa tor, FPL 64176 (2 mu M), enhanced the Ba2+ current in a voltage-depend ent manner. When cells were held at -80 mV, the saturating concentrati on (10 mu M) of nifedipine blocked similar to 11% of the control Ba2current. The major component of the Ca2+ channels was N type (63%), wh ich was blocked by a saturating concentration (1 mu M) of omega-conoto xin GVIA. Approximately 19% of the control Ba2+ current was sensitive to omega-conotoxin MVIIC (5 mu M) but insensitive to low concentration s (30 and 100 nM) of omega-agatoxin IVA (omega-Aga IVA). In addition, a high concentration (1 mu M) of omega-Aga IVA occluded the effect of omega-conotoxin MVIIC. Taken together, these results indicate that the omega-conotoxin MVIIC-sensitive current represents only the Q type of Ca2+ channels. The current that was insensitive to nifedipine and var ious toxins represents the R-type current (7%), which was sensitive to 100 mu M NiCl2. In conclusion, the intracardiac neurons from adult ra ts express at least four different subtypes (L, N, Q, and R) of HVA Ca 2+ channels. This information is essential for understanding the regul ation of synaptic transmission and excitability of intracardiac neuron s by different neurotransmitters and neural regulation of cardiac func tions.