Dissociation of the contractile and hypertrophie effects of vasoconstrictor prostanoids in vascular smooth muscle

To more clearly define the physiologic roles of thromboxane (TX)A₂ and primary prostaglandins (PG) in vascular tissue we examined vascular contractility, cell signaling, and growth responses. The growth-promoting effects of (15S)-hydroxy-11α,9α-(epoxymethano)prosta-5Z,13E-dienoic acid (U46619; TXA₂ agonist), PGF_2α, and PGE₂ consisted of protein synthesis and proto-oncogene expression, but not DNA synthesis or cell proliferation. U46619 contracted rat aortas and increased cultured rat aortic vascular smooth muscle cell intracellular free calcium concentration [Ca²⁺]_i, [³H]inositol monophosphate (IP) accumulation, myosin light chain phosphorylation, and protein synthesis ([³H]leucine incorporation) with EC₅₀ values ranging from 10 to 50 nM. Each of these responses was inhibitable with the TXA₂ receptor antagonist [1S] 1α,2β(5Z),3β,4α-7-(3-{2-[(phenylamino)carbonyl]hydrazino} methyl)-7-oxabicyclo[2.2.1]hept-2-yl-5-heptenoic acid (SQ29548). In contrast, PGF_2α increased [Ca²⁺]_i, [³H]IP, and protein synthesis with EC₅₀ values of 30-230 nM but contracted rat aortas with an EC₅₀ of 4800 nM. PGE₂ increased [Ca²⁺]_i, [³H]IP accumulation, protein synthesis, and contracted rat aortas with EC₅₀ values of 2.5-3.5 μM. TXA₂ receptor blockade prevented PGF_2α- and PGE₂-induced aortic contraction and cell myosin light chain phosphorylation, but not cell signaling or protein synthesis. Binding studies to vascular smooth muscle TXA₂ receptors using 1S-[1α,2β(5Z),3α(1E,3S),4α]-7-{3-[3-hydroxy-4-(p-[ ¹²⁵I] iodophenoxy)-1-butenyl]7-oxabicyclo[2.2.1]hept-2-y1}-5-heptenoic acid ([¹²⁵I]BOP) showed U46619, SQ29548, PGF_2α, and PGE₂ competition for TXA₂ receptor binding at concentrations similar to their EC₅₀ values for aortic contraction, while binding competition with [³H]PGF₂₀ and [³H]PGE₂ demonstrated the specificity of [¹²⁵I]BOP and SQ29548 for TXA₂ receptors. The results suggest that 1) PGF_2α- and E₂-stimulated vessel contraction is due to cross- agonism at vascular TXA2 receptors; 2) PGF_2α stimulates TXA₂ receptor-independent vascular smooth muscle protein synthesis at nanomolar concentrations, consistent with an interaction at its primary receptor; and 3) TXA₂ is a potent stimulus for vascular smooth muscle contraction and protein synthesis. We suggest that the main physiologic effect of PGF_2α may be as a stimulus for vascular smooth muscle cell hypertrophy, not as a contractile agonist.