Andrzej M. Janczewski

Modulation of sarcoplasmic reticulum Ca2+ cycling in systolic and diastolic heart failure associated with aging

Hypertension, atherosclerosis, and resultant chronic heart failure (HF) reach epidemic proportions among older persons, and the clinical manifestations and the prognoses of these worsen with increasing age. Thus, age per se is the major risk factor for cardiovascular disease. Changes in cardiac cell phenotype that occur with normal aging, as well as in HF associated with aging, include deficits in ß-adrenergic receptor (ß-AR) signaling, increased generation of reactive oxygen species (ROS), and altered excitation–contraction (EC) coupling that involves prolongation of the action potential (AP), intracellular Ca2+ (Cai2+) transient and contraction, and blunted force- and relaxation-frequency responses. Evidence suggests that altered sarcoplasmic reticulum (SR) Ca2+ uptake, storage, and release play central role in these changes, which also involve sarcolemmal L-type Ca2+ channel (LCC), Na+–Ca2+ exchanger (NCX), and K+ channels. We review the age-associated changes in the expression and function of Ca2+ transporting proteins, and functional consequences of these changes at the cardiac myocyte and organ levels. We also review sexual dimorphism and self-renewal of the heart in the context of cardiac aging and HF.

Overexpression of glucagon-like peptide-1 receptor in an insulin-secreting cell line enhances glucose responsiveness.

Glucagon-like peptide-1 (GLP-1), secreted from intestine in response to food intake, enhances insulin secretion from pancreatic beta-cells. In this study, we evaluated the effects of stably transfecting the GLP-1 receptor into an insulinoma cell line, RIN 1046-38, on basal and glucose-mediated insulin secretion and on second messenger pathways involved in insulin secretion. The GLP-1 receptor transfected cells had similar insulin mRNA levels but higher insulin content compared with parental cells. In GLP-1 receptor transfected cells, glucose (0.5 mM)-mediated insulin release was increased compared with parental cells (4.52 +/- 0.79 pmol insulin/l per mg protein x h vs. 2.21 +/- 0.36 pmol insulin/l per mg protein x h; mean +/- S.E., n = 6, P = 0.015, in transfected vs. parental cells, respectively). By hemolytic plaque assay measuring single cell insulin secretion, we observed that in the GLP-1 receptor transfected cells versus parental cells the increased insulin secretion was due to the presence of more glucose-responsive cells as well as more insulin released in response to glucose per cell. Resting intracellular cAMP was higher in the GLP-1 transfected cells (35.96 +/- 3.88 vs. 18.6 +/- 2.01 nmol/l per mg protein x h; mean +/- S.E., n = 4, P = 0.039, in transfected vs. parental cells, respectively). In response to GLP-1, both GLP-1 receptor transfected cells and parental cells showed increased cAMP levels independent of glucose. Resting intracellular calcium was the same in both parental and GLP-1 receptor transfected cells. However, more cells were responsive to glucose in the GLP-1 receptor transfected cells and the calcium transients attained in the presence of glucose developed at a faster rate and reached a higher amplitude than in parental cells. We conclude that having an excess of GLP-1 receptors renders beta-cells more sensitive to glucose.

GLUCAGON-LIKE PEPTIDE-1 DOES NOT MEDIATE AMYLASE RELEASE FROM AR42J CELLS

In this study, AR42J pancreatic acinar cells were used to investigate if glucagon‐like peptide‐1 (GLP‐1) or glucagon might influence amylase release and acinar cell function. We first confirmed the presence of GLP‐1 receptors on AR42J cells by reverse trasncriptase‐polymerase chain reaction (RT‐PCR), Western blotting, and partial sequencing analysis. While cholecystokinin (CCK) increased amylase release from AR42J cells, GLP‐1, alone or in the presence of CCK, had no effect on amylase release but both CCK and GLP‐1 increased intracellular calcium. Similar to GLP‐1, glucagon increased both cyclic adenosine monophosphate (cAMP) and intracellular calcium in AR42J cells but it actually decreased CCK‐mediated amylase release (n = 20, P < 0.01). CCK stimulation resulted in an increase in tyrosine phosphorylation of several cellular proteins, unlike GLP‐1 treatment, where no such increased phosphorylation was seen. Instead, GLP‐1 decreased such protein phosphorylations. Genestein blocked CCK‐induced phosphorylation events and amylase secretion while vanadate increased amylase secretion. These results provide evidence that tyrosine phosphorylation is necessary for amylase release and that signaling through GLP‐1 receptors does not mediate amylase release in AR42J cells. J. Cell. Physiol. 181:470–478, 1999. Published 1999 Wiley‐Liss, Inc.

The effect of prolonged rest on calcium exchange and contractions in rat and guinea-pig ventricular myocardium

The exchange of calcium in isolated, perfused rat ventricular myocardium was measured by means of 45Ca2+ and compared with that in guinea-pig. In the rat the total exchangeable calcium pool containing 0.84 +/- 0.03 mmol/kg of wet weight (w.w.) consists of at least two fractions: excitation-dependent, beat-to-beat exchanging fraction of about 0.1 mmol/kg w.w. which is lost at rest and re-gained during post-rest contractions; a fraction containing 0.74 +/- 0.04 mmol/kg w.w. of Ca2+ exchanging both at rest and during stimulation. During prolonged rest the rat ventricular myocardium loses only 13% (0.1 mmol/kg w.w.) of its exchangeable calcium whereas in the ventricle of guinea-pig heart 72% (0.94 mmol/kg w.w.) Ca2+ is lost at rest. These differences conform to the differences in the response of the contractile force to prolonged rest: the first post-rest contraction of the rat ventricle is stronger than the steady-state beats whereas the guinea-pig ventricular muscle loses its contractility. These results are compatible with the hypothesis that the calcium fraction released from resting guinea-pig ventricle is an important component of the mechanism of the slow force-frequency relationship.

Mechanisms of Relaxation: Perspectives from Studies in Single Cardiac Cells

Sarcomere shortening and relaxation in heart cells depend upon myofilament interaction, that is, crossbridge formation and dissociation between actin and myosin, which is regulated in a complex manner by numerous mechanico-chemical factors (Figure 16-1). These factors include the extent of Ca2+ binding to troponin C, concentrations of ATP, inorganic phosphate, Mg2+ and H+, isoforms of myosin and troponins, the phosphorylation status of some of these, sarcomere length, and the mechanical load borne [1]. Intracellular loading factors, that is, forces, are generated within sarcomeres and opposed by other intracellular structures, and other tissue loading factors are generated by intercellular connections and extracellular matrix, for example, by the amount, type, and geometry of collagen network.

Changes in the Heart That Accompany Advancing Age: Humans to Molecules

Age per se is the major risk factor for cardiovascular disease. Elucidation of the age-associated alterations in cardiac and arterial structure and function at both the cellular and molecular levels provides valuable clues that may assist in the development of effective therapies to prevent, to delay, or to attenuate the cardiovascular changes that accompany aging and contribute to the clinical manifestations of chronic heart failure. Changes in cardiac cell phenotype that occur with normal aging, as well as in HF associated with aging, include deficits in β-adrenergic receptor (β-AR) signaling, increased generation of reactive oxygen species (ROS), and altered excitation–contraction (EC) coupling that involves prolongation of the action potential (AP), intracellular Ca2+ (Cai 2+) transient and contraction, and blunted force and relaxation-frequency responses. Evidence suggests that altered sarcoplasmic reticulum (SR) Ca2+ uptake, storage, and release play central role in these changes, which also involve sarcolemmal L-type Ca2+ channel (LCC), Na+−Ca2+ exchanger (NCX), and K + channels.