Stereoselective modulation of ryanodine-sensitive calcium channels by the delta isomer of hexachlorocyclohexane (delta-HCH).
delta-Hexachlorocyclohexane (delta-HCH) is shown to be 30-fold more potent as a positive inotropic agent with rat atrial strips compared with lindane (gamma-HCH). Threshold and ED50 values for enhanced contractile force at a pacing frequency of 0.5 Hz are less than 1 microM and 2.2 microM for delta-HCH and 40 microM and 63 microM for gamma-HCH, respectively. Contracture developed in atria exposed to greater than 4 microM delta-HCH (ED50 = 11 microM) but not in atria exposed to gamma-HCH. Uptake and release of Ca++ measured from actively loaded cardiac sarcoplasmic reticulum (SR) vesicles is measured with antipyrylazo III. Although delta-HCH (30 microM) decreases Ca(++)-dependent ATPase by 20%, it does not significantly alter Ca++ loading in the presence of ruthenium red. Addition of delta-HCH (5-50 microM) after loading is complete causes rapid, dose-dependent release of Ca++ from SR. Ca++ release induced by delta-HCH is markedly stereoselective. Compared with gamma-HCH (50 microM), delta-HCH (50 microM) induces a nearly 20-fold higher initial rate of Ca++ release (4.3 nmol of Ca++/mg/sec). Studies with [3H]ryanodine demonstrate that delta-HCH sharply inhibits Ca(++)- or daunorubicin-activated radioligand binding (IC50 = 37 and 25 microM, respectively, logit slope = 2). Inhibition of [3H]ryanodine-binding by delta-HCH is stereoselective inasmuch as IC50 values for alpha, beta and gamma isomers are greater than 100 microM. The delta-HCH modified Ca++ channel appears to proceed by a noncompetitive mechanism (reducing Bmax in equilibrium experiments) with respect to the conformationally sensitive binding site for [3H]ryanodine.(ABSTRACT TRUNCATED AT 250 WORDS)
Pessah
I N
IN
Department of Veterinary Pharmacology and Toxicology, University of California, Davis.
Mohr
F C
FC
Schiedt
M
M
Joy
R M
RM
D000251:Adenosine Triphosphatases / Q000037:antagonists & inhibitors; D000818:Animals; D001665:Binding Sites; D001692:Biological Transport; D002118:Calcium / Q000378:metabolism; D015220:Calcium Channels / Q000187:drug effects*; D005260:Female; D001556:Hexachlorocyclohexane / Q000494:pharmacology*; D066298:In Vitro Techniques; D008297:Male; D009200:Myocardial Contraction / Q000187:drug effects; D051381:Rats; D011919:Rats, Inbred Strains; D012433:Ryanodine / Q000494:pharmacology*; D012519:Sarcoplasmic Reticulum / Q000378:metabolism; D013237:Stereoisomerism
D015220:Calcium Channels; D012433:Ryanodine; D001556:Hexachlorocyclohexane; D000251:Adenosine Triphosphatases; D002118:Calcium