EFFECTS OF ALPHA-ADRENERGIC STIMULI ON MESENTERIC COLLECTING LYMPHATICS IN THE RAT

Effects of a-adrenergic stimuli on mesenteric collecting lymphatics in the rat. Am. J. Physiol. 273 (Regulatory Integrative Comp. Physiol. 4 2): R331-R336, 1997.-The present study examined the effects of alpha(1 )- and alpha(2)-adrenergic stimuli on rat mesenteric collecting lympha tics in vivo. Sprague-Dawley rats were anesthetized, and the mesentery was prepared for intravital microscopic study. Mesenteric collecting lymphatic diameter was continuously monitored by using a computerized video tracking system, and indexes of lymphatic pumping (e.g., contrac tion frequency, stroke volume, ejection fraction, and muscle shortenin g velocity) were determined from the diameter record. Contractile acti vity was monitored before and during the administration of various adr energic agonists and antagonists. The receptor antagonists prazosin (a lpha(1)) and yohimbine (alpha(2)) did not significantly alter baseline diameter or contractile activity, which suggests that lymphatics poss ess no basal adrenergic tone. Norepinephrine and phenylephrine (0.1-1. 0 mu M) produced dose-dependent increases in frequency and decreases i n diameter. Lymphatic pump flow increased in direct proportion to freq uency, because stroke volume did not change. The changes in lymphatic pumping produced by 1 mu M norepinephrine were completely blocked by p razosin or phentolamine and only partially blocked by yohimbine. The a lpha(2)-adrenoceptor agonist (alpha-methyl-norepinephrine) produced no changes in lymphatic activity. This latter observation suggests that a role for postjunctional alpha(2)-adrenoceptors in modu lating mesent eric lymphatic smooth muscle is unlikely. The results of these studies support the existence of alpha-adrenoceptors on lymphatic smooth musc le. It is concluded that conditions characterized by increased sympath etic outflow may augment lymphatic function through alpha(1)- but not alpha(2)-adrenoceptors.