Local modulation of neurotransmitter release in bovine splenic vein

Bovine splenic vein has an abundant sympathetic innervation. Isolated strips were used to examine whether autoinhibition of norepinephrine release from the noradrenergic nerve terminals could be demonstrated under various experimental conditions and whether additional local regulatory modulators of transmitter release could also be implicated. In particular, the possibility of a histamine interaction with presynaptic inhibitory receptors was examined because ultrastructural evidence disclosed a close spatial relationship between mast cells and noradrenergic nerve terminals in the vessel wall. To investigate the presence of presynaptic alpha-receptors the competitive blocking agent phentolamine was included in the superfusion medium at concentrations ranging from 1 to 50 microM during electrical field stimulation at frequencies between 1 and 10 Hz. Transmitter outflow was measured as fractional tritium release. Low frequency stimulation (1 Hz) with 1 microM phentolamine resulted in the typical increase in norepinephrine release characteristics for presynaptic alpha-receptor inhibition. In contrast, high frequency (10 Hz) stimulation in the presence of 50 microM phentolamine caused an unexpected decrease in norepinephrine outflow. This unusual result can be explained by additional pharmacological actions of phentolamine unrelated to alpha-receptor blockade, e.g. histamine release from the mast cells which subsequently can act on presynaptic inhibitory histamine receptors. This effect, manifested at higher phentolamine concentrations, would overcome the alpha-receptor blockade. The presence of histamine receptors was supported by the results from electrical stimulation in the presence of exogenous histamine. Histamine decreased norepinephrine outflow while increasing basal tension and the contractile response of the vein strip. Unexpectedly, these effects appeared to be mediated by histamine receptors of the H1-type because they were reduced after pyrilamine but unaffected by agonists and antagonists to receptors of the H2-type. It is speculated that interactions between mast cells and noradrenergic nerve terminals may serve to maintain homeostasis in the bovine splenic vein.