Calcium release by noradrenaline from central sarcoplasmic reticulum in rabbit main pulmonary artery smooth muscle

The subcellular composition of relaxed and noradrenaline-contracted rabbit main pulmonary artery smooth muscle cells was measured by electron probe X-ray microanalysis of cryosections of rapidly frozen tissue. Some of the preparations were made permeable with saponin and exposed to a known free Ca ion concentration, rapidly frozen, freeze-substituted, and also analysed by electron probe X-ray microanalysis. 98% of intracellular K could be replaced by Rb. This was done to remove the K peak that partially overlaps the Ca peak in the X-ray spectra. The final [Rb]i plus residual [K]i was not significantly different from the [K]i of normal tissue. The [Ca]i in Rb-containing tissue was not significantly different from the [Ca]i in normal, K-containing tissue. Non-mitochondrial micro-regions containing high [Ca] (up to 33 mmol/kg dry wt.) were found at sites 200 nm or more away from the plasma membrane. These micro-regions also contained high [P]. We consider the identification of these regions containing high [Ca] as sarcoplasmic reticulum (s.r.), validated by: (a) conventional electron micrographs that show no other structures in main pulmonary artery smooth muscle in sufficient quantity and location to account for the frequency of these regions, (b) the previous localization of strontium, a functional calcium analogue, in the central s.r. in these smooth muscles (Somlyo & Somlyo, 1971 a), (c) the present demonstration that the central s.r. in this tissue can accumulate large amounts of calcium oxalate. The proportion of regions containing high [Ca] (greater than 12.0 mmol/kg dry wt.) was significantly higher in relaxed (35 of 330 measurements) than in the contracted (14 of 337) tissues (P less than 0.005), or 26 of 34 vs. 6 of 31 high [Ca] measurements in regions identified as s.r. through their high phosphorus content (P less than 0.006). This difference is thought to represent Ca release from the central s.r. There was no significant difference (P greater than 0.05) between the distributions of P in relaxed and contracted smooth muscle. The total cell [Ca]i in relaxed Rb-containing tissue, measured with randomly positioned small probes (3.6 mmol/kg dry wt.), was the same as that measured with large defocused probes, indicating the validity of random sampling. A mathematical model was used to estimate the frequency of including s.r. (35 nm diameter and 5% of cell volume) by a randomly positioned electron probe (50 nm), because we could not visualize s.r. in the cryosections.(ABSTRACT TRUNCATED AT 400 WORDS)