Voltage-gated Ca channels are crucial for controlling fast Ca signalling during neurosecretion. In the chromaffin cells of adrenal medulla, the autocrine action of neurotransmitters and the production of diffusable second messengers, such as nitric oxide (NO), can induce drastic changes to Ca channel gating and to the Ca-dependent events controlling catecholamine secretion and cell activity. A functionally interesting Ca signalling pathway in these cells is the G_i/G_o protein-dependent inhibition of N- and P/Q-channels, which is activated by the neurotransmitters released during exocytosis (ATP and opioids). This feedback modulation reveals peculiar features at the single channel level (delayed activation at low voltages and fast activation after strong prepulses), which can be used as fingerprint to detect local secretion in membrane patches where Ca channels, G_i/G_o proteins and membrane autoreceptors are closely packed (Carabelli et al. 1998). Alternative Ca signalling pathways involve the multiple modulation of L-channels which can either up- or downregulate the gating activity of the channel. Down-modulation of L-channels can be induced either by G_i/G_o proteins activated by membrane autoreceptors (purinergic, opioidergic and adrenergic) (Carabelli et al. 2001) or by elevations of cGMP associated to the production of NO and the activation of PKG (Carabelli et al. 2002). The two mechanisms act in parallel and cause a marked reduction of L-channel gating activity, mainly by causing prolonged shut times, with no changes to mean open times, the latency to first channel opening and single channel conductance. At variance with the down-modulation of N- and P/Q-channels, both actions are voltage independent and do not cause substantial changes to the activation-inactivation kinetics of voltage-gated Ca currents.

L-channel gating can also be upregulated by autocrine pathways, mediated by cAMP/PKA and capable of increasing the probability of channel openings by enhancing the mean open time and lowering the mean shut times (Carabelli et al. 2001). Preliminary experiments in rat chromaffin cells indicate that Ca currents are under the control of β₁– and β₂-adrenoreceptors (β₁-ARs, β₂-ARs) stimulation. The cAMP/PKA-mediated upregulation of L-channels is primarily mediated by β₁-ARs (Carbone et al. 2001), through a remote mechanism of action, while the β₂-ARs are mainly coupled to PTX-sensitive G_i/G_o proteins and produce only localised inhibition.

Given that all these Ca signalling pathways are primarily autocrine (including NO production) and that the packed array of chromaffin cells in the adrenal medulla favours the accumulation of released material, it is evident that the distinct forms of Ca channel modulation described here widen the dynamic range by which chromaffin cells control their secretory activity at rest or during maximal frequency stimulation.

All procedures accord with current National guidelines.