Abstract 1019: Blood Flow Conditions Resistance Artery Smooth Muscle
Activation of the myosin phosphatase (MP) by the cGMP-dependent protein kinase (cGK) is a key step in NO/cGMP-mediated smooth muscle relaxation. The leucine zipper (LZ) mediated heterodimerization of the MP targeting subunit (MYPT1) with cGK is required for activation of MP. We have proposed that expression of MYPT1 LZ+/− isoform variants sets the sensitivity of smooth muscle to NO/cGMP mediated relaxation. We previously showed in a model of portal hypertension (high cardiac output/low vascular resistance) dynamic switching of the MA1 (1st order mesenteric artery) from MYPT1 LZ− to LZ+. We hypothesized that altered blood flow may condition vascular smooth muscle (VSM) and thereby determine vasodilator responses. We adapted the model of DeMey, in which adjacent MA1s are exposed to chronically elevated (HF) or reduced blood flow (LF) by ligation of MA2s (2nd order MA) of every other MA1. MYPT1 protein was down-regulated within 1 day of HF and LF conditions to less than 10% of control values along with a switch to the LZ+ isoform. In the HF arteries this was transient, with a switch back to control values by 28 days. In contrast in the LF arteries there was a continued transition to LZ+ isoform along with a return of MYPT1 protein levels at 28 days. After inhibition of endogenous NO with L-NAME, phenylephrine activated LF MA1 was more sensitive to SIN-1 (EC50: LF, n = 4, 0.7 ± 0.5 μM vs. HF, n = 5, 3.2 ± 1.3 μM vs. control, n = 5, 3.9 ± 1.1 μM, P<0.01). Similarly, the LF MA1 is more sensitive to addition of exogenous cGMP. The time for the vessel to reach 50% maximal relaxation (t1/2) induced by myosin kinase inhibitor ML-9 (25 μM) was 199± 23 sec in HF MA1, 149 ± 21 sec in LF MA1 and 159 ± 24 sec in control MA1 (n=4 each, P>0.05). SIN-1 at 1 μM added to ML-9 significantly reduced t1/2 (58 ± 7 sec) in LF arteries but not in the HF (135±27 sec) and control (141±2 sec) MA1 (n=3 each, P<0.05). We propose that flow conditions the resistance artery VSM, and that changes in MYPT1 expression in response to altered flow functions as a rheostat to set the level of sensitivity of VSM to dilator signals in order to normalize flow rates.