Endogenous activator of intestinal guanylate cyclase. It stimulates this enzyme through the same receptor binding region as the heat-stable enterotoxins. May be a potent physiological regulator of intestinal fluid and electrolyte transport. May be an autocrine/paracrine regulator of intestinal salt and water transport. This is the function of Ensembl gene identifier ENSG00000044012 (GUCA2B, guanylate cyclase activator 2B).