SYMPATHETIC IMPULSE REQUIREMENTS FOR PROTEIN SECRETION FROM RAT SUBMANDIBULAR GLANDS DIFFER FOR THE 2 MAIN CELL-TYPES

Recent studies concerning the roles of sympathetic nerves in the secre tion of proteins from the two main types of secretory cells in rat sub mandibular glands - acini and granular tubules - have revealed amazing differences in their responsiveness. Morphometric studies after prolo nged stimulation of the sympathetic nerve supply at 50 Hz in bursts of Is every 10s for 1 hour showed a 47% depletion of acinar granules and a 52% depletion from granular tubules but the latter was much more ir regular. On the other hand 1 hour of continuous parasympathetic stimul ation at 10 Hz caused no detectable degranulation from either cell typ e. Subsequently the effects of graded sympathetic stimulations were as sessed on a background of parasympathetic stimulation at 4Hz, in order to provide sufficient saliva in sequential samples of short duration to test for peroxidase secreted by acini and kallikrein from granular tubules. Addition of low frequency sympathetic stimulation from 0.1 Hz upwards immediately caused a gradually increasing secretion of peroxi dase that peaked at 2 Hz of continuous stimulation and was not increas ed by higher frequency stimulation applied in bursts, In complete cont rast low frequency sympathetic stimulation did not increase the secret ion of kallikrein which required high frequency stimulation and then i t was secreted explosively. Finally? sequential samples of saliva form ed in response to intermittent sympathetic stimulation at 50 Hz delive red in bursts for periods of up to 2 min, with rest pauses of 2 min be tween, were tested to study how the secretory events relate to time. T his protocol of intermittent stimulation incidentally produced a far g reater flow of sympathetic saliva than had been achieved previously wi th ongoing stimulation. The pattern of peroxidase secretion from the a cini remained steady throughout at moderate levels, whereas the output of kallikrein from granular tubules was dramatic in initial samples t hen steadily decreased, The glands showed an 84% depletion of kallikre in in a mere 9 min of such stimulation. with a corresponding loss of g ranules from the tubule cells but only a small reduction had occurred from the acinar cells. Thus. acinar cells require only low frequency s ympathetic stimulation to secrete their protein at optimal rates and t his can continue indefinitely. However, the granular tubules require h igh frequency sympathetic drive to secrete their prepackaged proteins and this occurs most efficiently when delivered in short sharp bursts. but the process is exhaustible. This dichotomy in their responsivenes s to sympathetic stimulation suggests that the two cell types in the s ame gland may be innervated by separate populations of adrenergic nerv es that fire at different rates.