The response of lymphatic smooth muscles to vasoactive substances

A study was made of the isotonic response of bovine mesenteric lymphatics to several physiological vasoactive substances. Contractions of lymphatic smooth muscles were induced by serotonin (5-HT), prostaglandin F2alpha (PGF2alpha), noradrenaline (NA), histamine, dopamine and acetylcholine (ACh). The smooth muscles were particularly sensitive to 5-HT. Excepting PGF2alpha no other substances could equal 5-HT in the magnitude of the maximum response. The majority of 5-HT receptors seemed to be the D receptors. The decreasing order of the contractile responses was as follows: 5-HT greater than PGF2alpha greater than NA greater than histamine greater than dopamine greater than ACh. The contractile response to ACh was observed only in specimens involving valvular region. It was very likely that, in the lymphatics, there were 2 kinds of receptors for catecholamines, i.e. alpha and beta receptors, and the stimulation of the former induced smooth muscle contraction and that of the latter relaxation. A difference was noticed between the responses of valvular and intervalvular segments to NA. Relaxations of lymphatic smooth muscles were induced not only by isoproterenol but also by adenosine and adenine nucleotides. The decreasing order of the relaxant responses was as follows: ISP greater than adenosine greater than ATP greater than ADP greater cyclic AMP greater than or equal to AMP. The relaxant responses to adenine nucleotides tended to reduce with decrease in the number of high energy phosphates.