PROBABILISTIC SECRETION OF QUANTA FROM VISUALIZED SYMPATHETIC-NERVE VARICOSITIES IN MOUSE VAS-DEFERENS

1. Sympathetic varicosities on the surface of smooth muscle cells of the mouse vas deferens were visualized with the fluorescent dye 3-3 Diethyloxardicarbocyanine iodide (DiOC2(5)) and quantal secretion recorded from t ese with both small diameter (4-6-mu-m) and large diameter (20-50-mu-m) microelectrodes. Small diameter electrodes were placed over one to three varicosities and large diameter electrodes over three to seven varicosities. 2. The size and distribution of varicosities along individual terminal branches was about the same when these were fluoresced with DiOC2(5) (length 1.09 +/- 0.40-mu-m (mean +/- S.D.): intervaricosity distance 5.53 +/- 2.68-mu-m) as when they were stained for catecholamines using Faglu fluorescence (length 1.05 +/- 0.43-mu-m; intervaricosity distance 5.12 +/- 2.79-mu-m) suggesting that DiOC2(5) does allow for identification of the catecholamine-containing varicosities. 3. The spontaneous excitatory junctional currents (EJCs) recorded from visualized varicosities with small diameter electrodes (amplitudes 59-67-mu-V) were much larger than those recorded with large diameter electrodes (amplitudes 25-29-mu-V). The frequency of evoked EJCs as well as the amplitude-frequency distribution of these EJCs varied greatly between sets of visualized varicosities recorded along individual branches. either with a small or large diameter electrode. These amplitude-frequency distributions typically followed Poisson statistics, in which the mean quantal content of the EJC (mBAR) varied by over threefold for different sets of varicosities on the same branch (mBAR was 0.07-0.21 for small electrodes whereas mBAR was 1-3 for large electrodes). 4. Although mBAR varied considerably for a constant number of varicosities beneath the electrode at different sites along a single branch, there was an overall correlation between mBAR and the number of varicosities, mBAR increasing on average 0.25 for each additional varicosity in a [Ca2+]o of 4.0 mM. 5. The frequency of evoked EJCs at visualized sets of varicosities along some branches was sufficiently high to allow binomial statistics to predict the amplitude-frequency distributions of evoked EJCs. In these cases mBAR was again shown to vary considerably along single terminal branches, and this was primarily due to variation in the probability of secretion (p) between sets of varicosities and not to variation in binomial parameter n. 6. In one case a relatively isolated varicosity, over 3-mu-m from adjacent varicosities, was recorded for 30 min with a 4-mu-m diameter electrode. The mean and variance of the evoked EJC was similar to that of the spontaneous EJCs suggesting that this varicosity secreted at most one quantum on arrival of the nerve impulse. 7. There was a correlation between the number of varicosities beneath the recording electrode and binomial parameter n. but not with the probability for secretion. p. Parameter n increased on average 0.8 for each additional varicosity.