Sympathetic impulse requirements for protein secretion from rat submandibular glands differ for the two main cell types

Recent studies concerning the roles of sympathetic nerves in the secretion of proteins from the two main types of secretory cells in rat submandibular glands - acini and granular tubules - have revealed amazing differences in their responsiveness. Morphometric studies after prolonged 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 irregular. On the other hand 1 hour of continuous parasympathetic stimulation at 10 Hz caused no detectable degranulation from either cell type. Subsequently the effects of graded sympathetic stimulations were assessed 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 peroxidase that peaked at 2 Hz of continuous stimulation and was not increased by higher frequency stimulation applied in bursts, In complete contrast low frequency sympathetic stimulation did not increase the secretion of kallikrein which required high frequency stimulation and then it was secreted explosively. Finally? sequential samples of saliva formed in response to intermittent sympathetic stimulation at 50 Hz delivered in bursts for periods of up to 2 min, with rest pauses of 2 min between, were tested to study how the secretory events relate to time. This protocol of intermittent stimulation incidentally produced a far greater flow of sympathetic saliva than had been achieved previously with ongoing stimulation. The pattern of peroxidase secretion from the acini remained steady throughout at moderate levels, whereas the output of kallikrein from granular tubules was dramatic in initial samples then steadily decreased, The glands showed an 84% depletion of kallikrein in a mere 9 min of such stimulation. with a corresponding loss of granules from the tubule cells but only a small reduction had occurred from the acinar cells. Thus. acinar cells require only low frequency sympathetic stimulation to secrete their protein at optimal rates and this can continue indefinitely. However, the granular tubules require high 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 responsiveness to sympathetic stimulation suggests that the two cell types in the same gland may be innervated by separate populations of adrenergic nerves that fire at different rates.