Circulating immunoreactive substance P in man.

Immunoreactive substance P (iSP) has been measured in plasma in 77 normal subjects and in 125 hospital patients. Factors affectingin-vitro degradation of iSP were studied.In vivo, iSP is degraded in the liver and its level in the circulation is independent of kidney excretory function.During insulin-induced hypoglycaemic stress and also during glucose-tolerance test, iSP in plasma decreased transiently. No circadian rhythm of iSP was observed, but in a study in sleeping volunteers episodic secretory bursts were seen, separated by one- to two-hour intervals, the first peak appearing about 90 minutes after the subjects fell asleep.In a patient with carcinoid metastases in the liver, an elevated level of iSP was found in the general circulation with a marked gradient at the hepatic venous effluent.