COMPARISON OF VESICULAR VOLUME AND QUANTAL SIZE IN BOVINE CHROMAFFIN CELLS

Electrochemical measurements of vesicular content released were compar ed with the morphometric measurements of vesicular size in bovine chro maffin cells. Cross-sectional vesicular diameters were determined from electron micrographs. Two methods were used to determine the frequenc y histograms of ''true'' vesicular diameters (i.e. diameters of the ve sicles in the equatorial plane): (i) ''peeling off'' method [Coupland R. E. (1968), Nature 217, 384-388], and (ii) summation of individual p robabilities of ''true'' vesicular diameters. Quantal size was estimat ed from the area under the spontaneous current spike detected electroc hemically. The frequency histograms of ''true'' vesicular diameters ar e found to be skewed (thus not well described by a Gaussian function) irrespective of the method used to calculate them, as are the frequenc y histograms of the cube roots of the quantal sizes. Furthermore, we a lso find that the frequency histograms of electrochemical measurements (the cube roots of quantal sizes) have lower skews and coefficients o f variation than those of morphometric measurements (''true'' vesicula r diameters), with discrepancy being especially pronounced for noradre naline-secreting cells. Such a difference in both coefficients of vari ation and skews suggests that the intravesicular catecholamine concent ration is not uniform, but that it is lower for vesicles of larger siz e. In conclusion a variety of factors-vesicular volume, vesicular surf ace area to volume ratio, binding capacity of chromogranin and/or ATP, likely determines the amount of catecholamine stored in the vesicle. (C) 1998 IBRO. Published by Elsevier Science Ltd.