Postnatal changes in caldesmon expression and localization in cardiac myocytes

Caldesmon is a heat-stable protein found in both muscle and non-muscle tissue. It binds to a number of contractile and cytoskeletal proteins and may be involved in regulating acto-myosin interaction in smooth muscle cells and/or the assembly of microfilaments in muscle and non-muscle cells. We have shown previously that caldesmon is localized at the Z-lines in adult cardiac myocytes and that both the low- and high-molecular-weight forms (l-caldesmon and h-caldesmon, respectively) are present in atrial and ventricular myocytes. Here we examined the expression of caldesmon and its localization in freshly isolated cardiac myocytes during postnatal development and when these myocytes were grown in culture. We found that l-caldesmon is expressed in both neonatal and adult rat ventricular myocytes. The expression of h-caldesmon, however, was not detected in myocytes from newborn animals but increased during the first 2 weeks of postnatal development. Caldesmon was generally not co-localized with α-actinin at the Z-lines in neonatal myocytes but became increasingly more so during the first 2 weeks of postnatal development. When myocytes from 5- and 10-day-old rats were grown in primary culture, h-caldesmon expression decreased and caldesmon could not be detected at the Z-lines in the cultured cells. These results indicate that caldesmon plays a role at the Z-lines in adult cardiac myocytes; however, its localization at the Z-lines is not necessary for the prenatal development that occurs at these sites or for the establishment of a contractile phenotype in cultured cardiac myocytes

Raloxifene acutely suppresses ventricular myocyte contractility through inhibition of the L-type calcium current

1. The selective oestrogen (ER) receptor modulator, raloxifene, is widely used in the treatment of postmenopausal osteoporosis, but may also possess cardioprotective properties. We investigated whether it directly suppresses myocyte contractility through Ca(2+) channel antagonism in a similar way to 17β-oestradiol. 2. Cell shortening and Ca(2+) transients were measured in single guinea-pig ventricular myocytes field-stimulated (1 Hz, 37°C) in a superfusion chamber. Electrophysiological recordings were performed using single electrode voltage-clamp. 3. Raloxifene decreased cell shortening (EC(50) 2.4 μM) and the Ca(2+) transient amplitude (EC(50) 6.4 μM) in a concentration-dependent manner. At a concentration of 1 μM, raloxifene produced a 33±2% (mean±s.e.m) and 24±2% reduction, respectively (P<0.001, n=14 for both parameters). 4. These inhibitory actions were not observed in myocytes that had been incubated with the specific antagonist, ICI 182,780 (10 μM) (n=11). 5. Raloxifene (1 μM) shortened action potential durations at 50 and 90% repolarisation (P<0.05 and <0.001, respectively; n=27) and decreased peak L-type Ca(2+) current by 45%, from −5.1±0.5 pA/pF to −2.8±0.3 pA/pF (P<0.001, n=18). 6. Raloxifene did not significantly alter sarcoplasmic reticulum Ca(2+) content, as assessed by integrating the Na(+)/Ca(2+) exchanger currents following rapid caffeine application. 7. The present study provides evidence for direct inhibitory actions of raloxifene on ventricular myocyte contractility, mediated through Ca(2+) channel antagonism