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Calcium channel binding characteristics in the human heart
M S Finkel1, R E Patterson, W C Roberts
1Department of Medicine, University of Pittsburgh, Pennsylvania 15213.
Insights
Researchers characterized calcium channel binding sites in human hearts using [3H]nitrendipine. They found a homogeneous distribution of high-affinity sites, with altered binding in hypertrophic cardiomyopathy, demonstrating direct study feasibility.
Area of Science:
- Cardiology
- Pharmacology
- Molecular Biology
Background:
- Calcium channels are crucial for cardiac function.
- Understanding their distribution and properties in human hearts is vital for therapeutic development.
- Previous studies often relied on animal models or indirect measures.
Purpose of the Study:
- To characterize the number, affinity, pharmacologic specificity, and regional distribution of calcium channel binding sites in human hearts.
- To investigate the feasibility of directly studying cardiac calcium channels in human tissue.
- To compare binding site characteristics in healthy hearts versus those with specific cardiac conditions.
Main Methods:
- Radioligand binding assays using [3H]nitrendipine.
- Scatchard analysis of saturation data from autopsy and surgical human heart specimens.
- Ligand competition experiments to assess pharmacologic specificity.
- Comparison of binding parameters in healthy, postmortem, and hypertrophic cardiomyopathy samples.
Main Results:
- Homogeneous distribution of high-affinity calcium channel binding sites (low KD, nanomolar range) across different human heart regions (atrium, ventricle, septum).
- Pharmacologic specificity confirmed with known calcium channel blockers (nifedipine, nitrendipine, verapamil) and diltiazem.
- Similar binding characteristics observed in fresh and up to 18-hour postmortem hamster hearts.
- Significantly increased binding capacity (Bmax) in a hypertrophic cardiomyopathy specimen compared to healthy controls.
Conclusions:
- Direct characterization of calcium channel binding sites in human hearts is feasible using radioligand assays.
- The study provides quantitative data on the distribution and affinity of these sites in healthy human myocardium.
- Findings suggest potential alterations in calcium channel binding in specific cardiac pathologies like hypertrophic cardiomyopathy, warranting further investigation.
Abstract:
The number, affinity, pharmacologic specificity and regional distribution of calcium channel binding sites in human hearts obtained at autopsy and open heart surgery were characterized using the radioligand [3H]nitrendipine. Scatchard analyses of saturation data from 6 autopsy hearts revealed a homogeneous distribution of high affinity binding sites (affinity-1 [KD] = 0.44 +/- 0.06, 0.52 +/- 0.07, 0.32 +/- 0.02, 0.30 +/- 0.03, and 0.45 +/- 0.01 nM; binding capacity [Bmax] = 30 +/- 4, 27 +/- 6, 25 +/- 7, 33 +/- 3, and 28 +/- 4 fmol/mg protein in right atrium, right ventricle, left atrium, left ventricle and ventricular septum, respectively). In ligand competition experiments, nifedipine and nitrendipine completely displaced binding with partial displacement by verapamil and 35% enhancement of binding by 10(-5) M diltiazem at 37 degrees. Analyses of right atrial appendages obtained at open heart surgery from 5 coronary artery bypass patients provided similar results (KD = 0.2 +/- 0.03 nM, Bmax = 42 +/- 2 fmol/mg protein). In addition, no significant differences in KD or Bmax were found in 3 hamster hearts assayed at the time of death or up to 18 hours postmortem at either 4 or 25 degrees. In contrast, there was a significant increase in Bmax (110 fmol/mg protein) with no change in KD (0.3 nM) in a myomectomy specimen from a patient with obstructive hypertrophic cardiomyopathy compared with either autopsy or surgical specimens. These studies illustrate the feasibility and potential advantages of studying calcium channels directly in human hearts.