MICROSCOPIC HETEROGENEITY IN UNITARY N-TYPE CALCIUM CURRENTS IN RAT SYMPATHETIC NEURONS

1. Single N-type calcium (Ca2+) channels in rat superior cervical gang lion neurons display complex patterns of activity in both inactivating and non-inactivating gating modes. Unitary currents were elicited by holding the patch at -90 mV and stepping to +30 mV for 740 ms. Barium( 110 mM) was used as the charge carrier. The dihydropyridine agonist ()-202-791 was included in the bath to ensure that single channel recor dings showed no L-type Ca2+ channel mode 2 activity. Using this protoc ol, we characterized three additional patterns of N-type Ca2+ channel activity named: (1) LLP for large unitary current amplitude (i = -0.92 pA) and low open probability (P-o = 0.26); (2) SLP for small unitary current amplitude (i = -0.77 pA) and low open probability (P-o = 0.25) ; and (3) XHP for its small unitary current(i = -0.77 pA) and higher o pen probability (P-o = 0.39). 2. Transitions among these patterns of a ctivity occur more slowly than transitions between closed and open sta tes, resulting in significant clustering of like sweeps. Thus, the com plicated gating of single N-type Ca2+ channels can be dissected into m ultiple, independent modes, each with the same reproducible pattern of activity. 3. This heterogeneous activity is not unique to sympathetic neurons, for inactivating (4), non-inactivating (4), XLP (4) and SHP (3 patches) gating modes were also observed in cell-attached patch rec ordings (n = 4) of single N-type Ca2+ channels in differentiated phaeo -chromocytoma (PC12) cells. 4. The 1568 sweeps from four single N-type Ca2+ channel recordings that used the same voltage protocol were cate gorized by mode to determine the frequency of occurrence of each. Of t he 54 % of sweeps that showed activity, 42 % were inactivating and 58 % were non-inactivating. The contribution by each mode to the sustaine d current was estimated using the equation: I = NP(o)i, where N is the frequency of occurrence of each mode and P-o and i are the mean value s of open probability and unitary current amplitude respectively. The LLP mode contributed 18 %, the SLP mode 16 %, and the SHP mode 66 % of the sustained whole cell N-type Ba2+ current. 5. The variability in t he incidence among these modes in other cell types may resolve some of the controversy surrounding the characterization of N- and L-type who le cell Ca2+ current components in peripheral neurons. In addition, th e number of different modes provides a source of plasticity that may b e a target of modulation by neurotransmitters and cellular signals.