Mechanisms of PTHrP-induced inhibition of smooth muscle contractility in the guinea pig gastric antrum.

Background: Parathyroid hormone-related protein (PTHrP) that causes hypercalcemia of malignancy appears to function as an endogenous smooth muscle relaxant. For example, PTHrP released upon bladder wall distension relaxes detrusor smooth muscle to accommodate urine. Here, we explored mechanisms underlying PTHrP-induced suppression of the smooth muscle contractility in the gastric antrum that also undergoes a passive distension.

Methods: Effects of PTHrP on phasic contractions and electrical slow waves in the antral smooth muscle of the guinea pig stomach were studied using isometric tension and intracellular microelectrode recordings, respectively. Fluorescent immunohistochemistry was also carried out to identify the distribution of PTH/PTHrP receptors.

Key Results: Parathyroid hormone-related protein (1-100 nM) reduced the amplitude of phasic contractions and the basal tension. N -nitro-l-arginine (L-NA, 100 μM), a nitric oxide (NO) synthase inhibitor, or 1H-[1,2,4]oxadiazolo-[4, 3-a]quinoxalin-1-one (ODQ, 10 µM), a guanylate cyclase inhibitor, diminished the PTHrP (10 nM)-induced reduction in the amplitude of phasic contractions. SQ22536 (300 μM), an adenylate cyclase inhibitor, attenuated the PTHrP-induced reduction in basal tension. The combination of ODQ (10 μM) and SQ22536 (300 μM) inhibited the PTHrP-induced reductions in both phasic contractions and basal tension. PTHrP (100 nM) had no inhibitory effect on the electrical slow waves in the antral smooth muscle. PTH/PTHrP receptors were expressed in cell bodies of PGP9.5-positive neurons in the myenteric plexus.

Conclusions & Inferences: Parathyroid hormone-related protein exerts its inhibitory actions on the antral smooth muscle via both nitric oxide-cyclic guanosine monophosphate (NO-cGMP) and cyclic adenosine monophosphate (AMP) pathways. Thus, PTHrP may act as an endogenous relaxant of the gastric antrum employing the two complementary signaling pathways to ensure the adaptive relaxation of stomach.