No abstract

The molecular basis for the systolic and diastolic dysfunction characteristic of end-stage heart failure in humans remains poorly understood. It has been proposed that both abnormal calcium handling and defects in the contractile apparatus may contribute to the myocardial dysfunction. Two channels, the calcium release channel (CRC) or ryanodine receptor of the sarcoplasmic reticulum (SR), and the slow calcium channel or dihydropyridine receptor (DHPR) of the transverse tubule, play key roles in regulating intracellular calcium concentration and in excitation-contraction (E-C) coupling in the heart. The DHPR serves as the voltage sensor and plasma membrane calcium channel resulting in activation of the CRC during E-C coupling in heart muscle. In this study, we investigated the levels of CRC expression in several forms of end-stage heart failure in humans. A cardiac CRC cDNA was cloned from rabbit and used as a probe for Northern blot analyses to determine mRNA levels in the left ventricles of normal (n=4) and cardiomyopathic (n=34) human hearts from patients undergoing cardiac transplantation. Compared with normal patients, patients with ischemic cardiomyopathy (n=18) showed a 28% decrease in CRC mRNA levels (p<0.025) and patients with idiopathic dilated cardiomyopathy (n=14) a nonsignificant 12% increase. In these same hearts, a-actin levels were unchanged in end-stage heart failure, as has been previously reported. This is the first report indicating that the expression of the CRC mRNA is abnormal in end-stage human heart failure. The decreased expression of the CRC found specifically in patients with ischemic myopathy (but not idiopathic dilated cardiomyopathy) may, in part, explain differences in calcium handling and response to therapy observed among patients with different forms of cardiomyopathy. Decreased CRC expression could be related to abnormal contractile function in cardiomyopathic muscle resulting from ischemic insult. (Circulation Research 1992;71:18-26) KEY WoRDs * idiopathic dilated cardiomyopathy channel * mRNA * ryanodine receptor T Nhe calcium release channel (CRC) or ryanodine receptor of the sarcoplasmic reticulum (SR) is a large (565-kd) protein that forms the foot structure associating the SR with the transverse tubule

Two types of calcium channels signal excitation-contraction (E-C) coupling in striated muscle: dihydropyridine receptors (DHPRs, voltage-gated L-type calcium channels on the transverse tubule) and ryanodine receptors (RyRs, calcium release channels on the sarcoplasmic reticulum). Sarcolemmal depolarization activates the DHPR; subsequently, the RyR is activated and releases calcium that activates muscle contraction. We show in the present study that expression of the E-C coupling calcium channels is upregulated during myogenic development in the rabbit. Skeletal and cardiac muscle isoforms of the following genes were examined: the DHPR alpha 1, alpha 2, beta, and gamma subunits and the RyR. Distinct cardiac and skeletal muscle-specific cDNAs were isolated, encoding each of the DHPR subunits and the RyR. The skeletal muscle DHPR alpha 1, alpha 2, beta, and gamma subunits and the cardiac DHPR alpha 1 subunit mRNA levels increased on the day of birth and at the adult stage compared with fetal levels. The skeletal and cardiac RyR mRNA levels increased on the day of birth and at adult stages compared with fetal levels. Ryanodine binding sites increased in both skeletal and cardiac muscle. We now provide a molecular explanation for the physiological "maturation" of the E-C coupling apparatus observed at the day of birth and during early postnatal development in both skeletal and cardiac muscles. Low levels of calcium channel expression in fetal cardiac and skeletal muscle make these tissues more sensitive to pharmacological therapy with calcium channel blockers, a phenomenon that has been reported in human neonates.

Elderly diabetics are a growing subset of the type 2 diabetic population with unique pathophysiologic characteristics and diabetic risk profiles. Therapeutic strategies that incorporate enhancement of GLP-1 action on beta-cells to improve beta-cell insulin secretion and glycemic control may be ideal for this distinct population and should be validated with further long-term clinical studies.

No abstract