Smooth muscle cells (SMC) were isolated from cat cerebral arteries and three sizes of pulmonary artery (less than 200-, 200- to 600-, and greater than 800-microns diameter) and used within 72-96 h. Change in cell length in response to hypoxia and other vasoactive agents was measured in a specially constructed cell chamber on an inverted microscope. Pulmonary artery SMC responded to hypoxia differently according to artery size. SMC from less than 200- and 200- to 600-microns-diameter pulmonary arteries shortened 18.6 +/- 4.85 and 24.2 +/- 2.70%, respectively. However, SMC from greater than 800-microns-diameter pulmonary arteries shortened 0.81 +/- 0.44%. Cerebral artery SMC plated on a flexible polydimethyl siloxane membrane showed loss of tension during exposure to hypoxia. The shortening of SMC from the 200- to 600-microns pulmonary arteries was accompanied by myosin phosphorylation. SMC from greater than 800-microns-diameter pulmonary arteries and cerebral arteries contained myosin that did not phosphorylate during hypoxia. The SMC from both artery types responded to norepinephrine, serotonin, prostaglandin F2 alpha and indomethacin and exhibited alpha-adrenergic receptor population patterns similar to those of intact arteries. The pattern of hypoxic responses exhibited by these nondedifferentiated pulmonary and cerebral artery SMC supports the idea that, at least in the cat, the hypoxic sensor is located within the SMC.

The fluorescent calcium indicator, fura 2, was used to test whether contraction of primary cultured smooth muscle cells (SMC) from small pulmonary arteries in response to hypoxia and the relaxation by large pulmonary and cerebral artery SMC were mediated by changes in cytoplasmic free Ca2+ (Ca2+c). Because SMC from large pulmonary and cerebral arteries contract to norepinephrine (NE), Ca2+c levels during NE exposure were measured to determine if they differed from those seen with hypoxia. Under hypoxic conditions, Ca2+c increased 64.1 +/- 11.1% above the normoxic baseline in small pulmonary artery SMC. In SMC from large pulmonary and cerebral arteries, Ca2+c decreased 25.2 +/- 9.20 and 28.1 +/- 5.80%. Ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA) and ryanodine attenuated the Ca2+c increase in the small pulmonary artery SMC. On exposure to NE, Ca2+c increased markedly in all three SMC types. EGTA and ryanodine treatment also attenuated NE-induced Ca2+c increases in all three SMC types. These results show that the three SMC types mobilize their available Ca2+ stores differently when exposed to hypoxia but similarly when exposed to NE. The data also suggest that a change in the Ca2+c concentration, rather than a change in the Ca2+ sensitivity of the contractile apparatus, is involved in the response to hypoxia.

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