The cardiovascular effects of vasoactive intestinal polypeptide (VIP) and substance P (SP) in vivo were studied in the Atlantic cod Gadus morhua. Special interest was focused on the distribution of blood to the gastrointestinal circulation.
VIP increased the blood flow to the gut by increasing cardiac output and by decreasing resistance in the vascular bed supplied by the coeliac artery. In addition, VIP had an inhibitory effect on spontaneous stomach motility.
SP induced a triphasic response in the coeliac artery blood flow. An initial increase was followed by a rapid decrease, to the control level or below, and a second increase in flow. The triphasic response was not changed after vagotomy, while atropine blocked the second phase, the decrease, indicating that a local cholinergic mechanism is involved. The significance of this temporary decrease in flow remains to be elucidated. SP also caused an increase in cardiac output and in mesenteric artery blood flow. In addition to the increase in cardiac output, the increase in gastrointestinal blood flow produced by SP is accomplished by a decreased resistance in the coeliac and mesenteric vascular beds.