Effects of IBMX on norepinephrine-induced vasoconstriction in small mesenteric arteries.

The present study assesses the effects of the phosphodiesterase inhibitor IBMX on norepinephrine (NE)-induced constriction of small mesenteric arteries. Arteries ( approximately 150 micrometer) were dissected from rats and mounted on a wire myograph for isometric force measurement. NE concentration effect curves were generated after exposure to 500 microM IBMX for 60 min. IBMX significantly reduced NE-induced tension development. Studies were also conducted following sarcoplasmic reticulum (SR) depletion (ryanodine, 10 microM) or L-type Ca2+ channel blockade [(+)-BAY K 8644, 10 microM] in the presence and absence of IBMX. Both SR depletion and L-channel blockade reduced NE-induced tension generation, consistent with incomplete Ca2+ mobilization. IBMX significantly attenuated NE responses in ryanodine and (+)-BAY K 8644-treated vessels. Finally, treatment of NE-stimulated vessels with IBMX (500 microM) caused a reduction in vascular tension that was greater than the concomitant reduction in cytosolic Ca2+ concentration ([Ca2+]i), indicating that a portion of the IBMX-mediated relaxation is Ca2+-independent. These data suggest that IBMX attenuation of NE responsiveness not only involves a reduction in [Ca2+]i but also a significant decrease in Ca2+ sensitivity.