THE CONTRIBUTION OF DIFFERENT TYPES OF CALCIUM CHANNELS TO ELECTRICALLY-EVOKED ADENOSINE RELEASE FROM RAT HIPPOCAMPAL SLICES

The role of L-, N- and P-type voltage-dependent calcium channels (VDCC s) in the release of adenosine from rat hippocampal slices was investi gated by evaluating the effect of the L-channel activator -5-nitro-5-[ 2-(trifluoromethyl)-phenyl]-3-pyridine carboxylic acid methyl ester (B ay K 8644) and of three calcium channel antagonists: the L-channel ant agonist nifedipine, the N-channel blocker omega-conotoxin GVIA (omega- CgTx) and the P-channel blocker omega-agatoxin IVA (omega-Aga-IVA). Ad enosine and inosine release, evoked by 5 min electrical stimulation at 10 Hz of hippocampal slices, was assayed by HPLC with ultraviolet abs orbance or fluorimetric detection. Nifedipine (100 nM) did not affect adenosine and inosine release evoked by electrical stimulation. Bay K 8644 (100 nM) brought about a statistically significant increase in ad enosine evoked release (70%). At a higher concentration (1 mu M) Bay K 8644 had no significant effect either on adenosine or inosine release evoked by electrical stimulation. The increase in adenosine release e licited by 100 nM Bay K 8644 was abolished by nifedipine (100 nM). Bot h omega-CgTx (10 mu M) and omega-Aga-IVA (200 nM) caused a statistical ly significant reduction (77-78%) in evoked release of adenosine. When the previously demonstrated glutamate-dependent component of the rele ase of adenosine was suppressed in the presence of the NMDA and non-NM DA receptor antagonists, D(-)-2-amino-7-phosphonoheptanoic acid (D-AP7 , 100 mu M) and 6,7-dinitroquinoxaline-2,3-dione (DNQX, 10 mu M), the remaining release of adenosine was again significantly reduced by omeg a-CgTx (10 mu M) (60%) and omega-Aga-IVA (200 nM) (73%). These data su ggest that, while L-type VDCCs are involved in the regulation of the e voked release of adenosine only when activated by Bay K 8644, both P- and N-channels play a direct role in the calcium entry involved in the coupling process between electrical stimulation and adenosine release .