In vivo arteriolar dilation in response to parathyroid hormone fragments

The in vivo responsiveness of small arterioles to the topical administration of two parathyroid hormone fragments was investigated using television microscopy. Male Sprague-Dawley rats were anesthetized with sodium pentobarbital (50 mg/kg) and second- and third-order arterioles in the cremaster muscle were exposed to increasing concentrations (2 X 10(-5) to 6 X 10(-4) mg/ml) of either hPTH (1-34) or bPTH-(3-34). Second- and third-order arterioles within the cremaster dilated (183% and 281% of control, respectively) following exposure to PTH-(1-34) in bath concentration of 10(-4) mg/ml and above. The dilation associated with PTH administration was abolished in second-order and greatly attenuated for third-order arterioles when the first two amino acid residues of the PTH molecule were removed (PTH (3-34) fragment). Inhibition of endogenous prostaglandins synthesis with mefenamic acid did not attenuate the vasodilator response to PTH. However, exposure to the muscarinic blocking agent atropine (10(-7) g/ml) totally inhibited the dilator response to PTH-(1-34). These data suggest that PTH induces arteriolar dilation by stimulation of muscarinic receptors in the vasculature possibly by causing the release of endogenous acetylcholine.