Contractile properties of periosteal arterioles in the guinea-pig tibia.

The periosteal arterioles of the compact bone may play a critical role in bone growth. To explore the contractile properties of tibial arterioles, spontaneous and nerve-evoked constrictions were compared in preparations from 3-week-old and 1-year-old guinea-pigs. Changes in arteriole diameters were measured using video microscopy. Their innervation was investigated using fluorescence immunohistochemistry. Fifty per cent and 40% of tibial arterioles from 3-week-old and 1-year-old guinea-pigs, respectively, exhibited spontaneous phasic constrictions that were inhibited by 1 μM nifedipine, 10 μM cyclopiazonic acid or 100 μM 2-APB. Nerve-evoked phasic constrictions in both age groups were largely suppressed by phentolamine (1 μM), an α-adrenoceptor antagonist, or sympathetic neurotransmitter depletion using guanethidine (10 μM) but were enhanced by spanttide (1 μM), a substance P receptor antagonist, or L-nitro arginine (L-NA; 100 μM), an inhibitor of nitric oxide synthase (NOS). Nerve-evoked constrictions in 1-year-old animals were smaller than those in younger animals but greatly enhanced by L-NA. Immunohistochemistry revealed sympathetic and substance P-positive primary afferent nerves running along the arterioles as well as endothelial NOS expression in both age groups. Spontaneous arteriolar constrictions appear to rely on both Ca release from the sarcoplasmic reticulum and Ca influx through L-type Ca channels. Noradrenaline released from sympathetic nerves triggers arteriolar constriction, while substance P released from primary afferent nerves dilates the arterioles by releasing nitric oxide (NO), presumably from the endothelium. Thus, the enhanced endothelial NO release in adult guinea-pigs may be important to increase the blood supply to meet the increased metabolic demands during bone growth.