Changes in sizes of the adrenaline-containing vesicles and their cores in frog cardiac sympathetic nerves after pharmacological treatments.

The sizes of adrenergic vesicles and their cores, as made visible by an acrylic aldehyde in sodium dichromate fixative, have been measured in electron micrographs of the sympathetic nerves amongst the frog's ventricular muscle. The animals were either normal or previously treated with drugs expected to affect the catecholamine content of the heart. The sympathetic nerves contain two overlapping populations of vesicles. A graphical method was used to separate these and determine the mean diameter of each population. The distribution of vesicles between the 'large' and 'small' populations is variable in normal animals. No changes could be detected in the experimental animals. The mean size of the 'large' vesicles is variable in normal animals. No changes could be detected in the experimental animals. Four injections of 5-hydroxydopamine caused a 5.8% increase in the diameter of the 'small' vesicles. No other treatment produced significant changes in vesicle size. Four injections of 5-hydroxydopamine caused a 50% increase in the diameter of the cores of the 'small' vesicles. Two injections of reserpine caused a 20% reduction in the diameter of the visible cores in the 'small' vesicles, and 34% of the vesicles lost their cores entirely. One injection of 6-hydroxydopamine or ten injections of alpha-methyl-tyrosine caused small reductions in 'small' core diameter. It is postulated that core formation in adrenergic nerves under these conditions is not solely dependent on their catecholamine content, but on this and another factor which may be part of the storage complex.