Contraction of rat vena cava by endothelin‐1 is dependent on phospholipase‐Cβ, but independent of IP₃ receptor activation

Abstract

In vascular smooth muscle, inositol 1,4,5‐trisphosphate (IP3) regulates excitation‐contraction coupling by activating the release of sarcoplasmic Ca2+ stores. IP3 is produced through the activity of phospholipase Cβ (PLCβ), which cleaves phosphatidylinositol into diacylglycerol (DAG) and IP3. While the vasoconstrictor ET‐1 activates PLCβ and increases IP3 formation in arteries, it is unclear if this is so in veins. We hypothesized that contraction to ET‐1 in rat aorta (RA) and vena cava (RVC) depends upon IP3‐mediated Ca2+ release. To test if PLCβ was activated by ET‐1, isometric contraction was measured in RA and RVC rings exposed to vehicle, the PLCβ inhibitor U‐73122 or its inactive analog U‐ 73343 (1 μM), or the IP3 receptor antagonist 2‐APB (100 μM). While U‐73343 did not significantly inhibit contraction to ET‐1, U‐73122 significantly reduced maximum contraction to ET‐1 in RA (86±6% of control) and RVC (49±11% of control). Contrary to our hypothesis, 2‐APB significantly reduced maximum contraction to ET‐1 only in RA (55±4%) and not RVC (82±8%). These data were supported by Western blot analysis, showing 63.1 times more IP3R‐1 and 12.6 times more IP3R‐2 protein in RA than RVC, as measured by densitometry. These findings suggest that ET‐1 does activate PLCβ in RA and RVC, but only contraction to ET‐1 in RA is regulated by IP3. Rather, DAG and not IP3 may regulate contraction to ET‐1 in RVC. Supported by NIH P01HL70687.