Mechanisms of ADRF release from rat aortic adventitial adipose tissue

Blood vessels are surrounded by variable amounts of adipose tissue. We showed earlier that adventitial adipose tissue inhibits rat aortic contraction by release of a transferable factor, adventitium-derived relaxing factor (ADRF), which activates smooth muscle K+ channels. However, little is known about the mechanisms of ADRF release. Using isolated rat aortic rings and isometric contraction measurements, we show that ADRF release depends on extracellular [Ca2+] (EC50 similar to 4.7 mM). ADRF effects do not involve neuronal presynaptic N-type Ca2+ and Na+ channels or vanilloid, cannabinoid, and CGRP receptors. ADRF release is strongly inhibited by the protein tyrosine kinase inhibitors genistein and tyrphostin A25. In contrast, daidzein, an inactive genistein analog, and the protein tyrosine kinase inhibitor ST638 had no effect. Protein kinase A inhibition by H89 also inhibited ADRF release, whereas the protein kinase G inhibitor KT-5823 had no effect. We propose that ADRF release is Ca2+ dependent and is regulated by intracellular signaling pathways involving tyrosine kinase and protein kinase A. Furthermore, ADRF release does not depend on perivascular nerve endings.