The effects of morphine on isolated cutaneous canine vascular smooth muscle

In order to evaluate the direct effects of morphine sulfate (MS) on vascular smooth muscle (VSM), a helical strip preparation of canine cutaneous VSM was prepared from vessels of approximately 150 to 250 mug internal diameter. When evaluated at a variety of muscle lengths from 20% to 60% above unstretched length, in the absence and in the presence of norepinephrine (2 to 10x10(-4) mg/cc), MS (10(-3) to 10(-1) mg/cc) failed to produce any noticeable effect. Thus, the well-known arteriolar dilator effect of MS is not a direct effect of the drug on VSM and it is not secondary to blockade of vascular alpha-adrenergic receptors.