CA2-INDUCED CA2+ RELEASE MEDIATES CA2+ TRANSIENTS EVOKED BY SINGLE ACTION-POTENTIALS IN RABBIT VAGAL AFFERENT NEURONS()

1. Standard intracellular recording techniques with 'sharp' micropipet tes were used to evoke action potentials (APs) in acutely dissociated adult nodose neurones. 2. APs induced a transient increase in [Ca2+](i ) (a calcium transient), recorded with fura-2, that was dependent upon [Ca2+](o) and the number of APs. Over the range of one to sixty-five APs, the relation between the amplitude of the calcium transient and t he number of APs was well fitted by a rectangular hyperbola (chi(2) = 3.53, r = 0.968). From one to four APs, the calcium transient-AP relat ion can be described by a line with a slope of 9.6 nM AP(-1) (r = 0.99 9). 3. Charge movement corresponding to Ca2+ influx evoked by a single AP was 39 +/- 2.8 pC (mean +/- S.E.M.) and did not change significant ly during trains of one to thirty-one APs (P < 0.05). 4. Caffeine (10 mM), a known agonist of the ryanodine receptor, produced an increase i n [Ca2+](i). The caffeine-induced rise in [Ca2+](i) was attenuated (by > 90%) by lowering [Cd2+](o), and by ryanodine (10 mu M), 2,5-di(t-bu tyl)hydroquinone (DBHQ, 10 mu M), or thapsigargin (100 nM). 5. Neurone s incubated with ryanodine, DBHQ or thapsigargin required at least eig ht APs to evoke a detectable calcium transient. These reagents did not significantly affect Ca2+ influx (P < 0.05). In the presence of these inhibitors, the calcium transient-AP relation exhibited slopes of 1.2 , 1.1 and 1.9 nM AP(-1) for ryanodine, DBHQ and thapsigargin, respecti vely. When compared with the slope of 9.6 nM AP(-1) in non-treated neu rones, it appears that Ca2+ influx produced by a single AP is amplifie d by ca 5- to 10-fold.