DIURNAL-VARIATION IN ADRENAL-GLAND FRESHWEIGHT DUE TO VASCULAR DAMMING - A POSSIBLE ROLE IN CORTICOSTERONE STORAGE

Previous in vitro studies have disclosed the existence of a diurnal va riation in adrenal gland freshweight in the minimally stressed rat (in active phase freshweight > active phase freshweight). In the present s tudy this active/inactive phase difference in adrenal weight was exami ned using light microscopy, transmission electron microscopy (TEM) and backscattered scanning electron microscopy (BSEM). Morphological and stereological examination of the resultant micrographs has shown a sig nificant increase in gland cross-sectional width in the active phase, localised to the zona fasciculata/reticularis region of the cortex. Ho wever, TEM examination of cells from this region, comparing volume and surface densities from active and inactive phase glands, has not prov ided evidence of a diurnal variation in cell size. Analysis of the BSE M investigation of vascular, cellular and interstitial compartments of the glands confirmed the absence of variation in the cellular compart ment but showed a diurnal variation in the vascular compartment of the zona fasciculata/reticularis. The circadian related changes in vascul ar volume density begin at the cortico-medullary border where greatest difference is observed between the active and inactive phases. This d ifference continues throughout the zona fasciculata/reticularis decrea sing in size as it approaches the zona glomerulosa region. These findi ngs are explained in terms of the existence of a cortico-medullary vas cular dam that is a possible contributor to the rapid steroidogenic re sponse seen on initial stimulation of the gland by adrenocorticotrophi c hormone (ACTH).