Guanine nucleotide‐ and inositol 1,4,5‐trisphosphate‐induced calcium release in rabbit main pulmonary artery.

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Abstract
1. The effects of activation of guanine nucleotide‐binding protein (G protein) by guanine nucleotides or sodium fluoride on the release of intracellular Ca2+ and on tension development were determined in chemically skinned strips of rabbit main pulmonary arteries (MPA). Ca2+ movements were monitored with Fura‐2, as the change in free Ca2+ concentration in the bath medium surrounding the skinned MPA. 2. Sodium fluoride or non‐hydrolysable analogues of GTP, guanosine 5'‐[gamma‐thio]triphosphate (GTP gamma S) and guanosine 5'‐[beta,gamma‐imido]triphosphate (GMP‐PNP), induced sustained and dose‐dependent contraction of skinned MPA. GTP (100 microM) induced transient contraction of skinned MPA. GTP gamma S did not contract intact MPA. We also confirmed that inositol 1,4,5‐trisphosphate (InsP3) released sufficient Ca2+ to induce contraction of skinned, but not intact, MPA. 3. Guanosine 5'‐[beta‐thio]diphosphate (GDP beta S), a non‐hydrolysable analogue of GDP that competitively inhibits the binding of guanine nucleotides to G proteins, inhibited the contractions induced by GTP gamma S. Neomycin (1 mM) inhibited the GTP gamma S‐induced contractions, but also, to a lesser extent, contractions induced by caffeine. 4. Depletion of Ca2+ from the sarcoplasmic reticulum (SR) or treatment with Triton X‐100 inhibited the GTP gamma S‐induced contractions. The effects of Ca2+ depletion was reversible, while that of Triton X‐100 was irreversible. GTP gamma S (up to 100 microM) had no apparent effect on the pCa‐tension curve of freeze‐glycerinated MPA. 5. GTP gamma S‐ or InsP3‐induced contractions occurred in the presence of 20 mM‐procaine, while this agent completely blocked the contraction induced by caffeine. 6. Both GTP gamma S and InsP3 induced an increase in the Fura‐2 fluorescence signal of the bath medium surrounding the skinned MPA, indicating that GTP gamma S releases intracellular Ca2+. The release of Ca2+ induced by GTP gamma S was inhibited by GDP beta S. 7. During the initial phasic contraction induced by GTP gamma S, added InsP3 had little or no additive effect, in contrast to its additive effect during the latter sustained contraction induced by GTP gamma S.(ABSTRACT TRUNCATED AT 400 WORDS)