Natalie Chandler

Molecular Architecture of the Human Sinus Node Insights Into the Function of the Cardiac Pacemaker

Background— Although we know much about the molecular makeup of the sinus node (SN) in small mammals, little is known about it in humans. The aims of the present study were to investigate the expression of ion channels in the human SN and to use the data to predict electrical activity. Methods and Results— Quantitative polymerase chain reaction, in situ hybridization, and immunofluorescence were used to analyze 6 human tissue samples. Messenger RNA (mRNA) for 120 ion channels (and some related proteins) was measured in the SN, a novel paranodal area, and the right atrium (RA). The results showed, for example, that in the SN compared with the RA, there was a lower expression of Nav1.5, Kv4.3, Kv1.5, ERG, Kir2.1, Kir6.2, RyR2, SERCA2a, Cx40, and Cx43 mRNAs but a higher expression of Cav1.3, Cav3.1, HCN1, and HCN4 mRNAs. The expression pattern of many ion channels in the paranodal area was intermediate between that of the SN and RA; however, compared with the SN and RA, the paranodal area showed greater expression of Kv4.2, Kir6.1, TASK1, SK2, and MiRP2. Expression of ion channel proteins was in agreement with expression of the corresponding mRNAs. The levels of mRNA in the SN, as a percentage of those in the RA, were used to estimate conductances of key ionic currents as a percentage of those in a mathematical model of human atrial action potential. The resulting SN model successfully produced pacemaking. Conclusions— Ion channels show a complex and heterogeneous pattern of expression in the SN, paranodal area, and RA in humans, and the expression pattern is appropriate to explain pacemaking.

The anatomy of the cardiac conduction system

All the myocytes within the heart have the capacity to conduct the cardiac impulse. A population of myocytes is specialized so as to generate the cardiac impulse and then to conduct it from the atrial to the ventricular chambers. This population has become known as the conduction system. Anatomists who seek to demonstrate the location of the components of this system must contend with the fact that the components of the system cannot be distinguished from the working myocardial elements by gross dissection. In important presentations to the German Pathological Society in 1910, rules were suggested for the histological distinction of these conducting cells. These rules proposed that the myocytes, to be considered as part of the conduction system, should be histologically discrete, traceable from section to section in serially prepared material, and if to be considered as tracts, should be insulated by fibrous tissue from the adjacent myocytes. Immunohistochemical techniques have now been developed that better demonstrate the distinction between the cells specialized to conduct from working myocytes. These new techniques, for the most part, confirm the accuracy of the initial descriptions. They also reveal additional areas with the characteristics of conduction tissues. These additional areas are located in a paranodal area adjacent to the sinus node, in the vestibules of both atrioventricular valvar orifices, and in a partial ring around the aortic root. In this review, we describe all these features, emphasizing the relationship of the newly recognized components to the established parts of the cardiac conduction system, and how the new findings need to be assessed in the light of the old criterions. Clin. Anat. 22:99–113, 2009.

Effects of streptozotocin-induced diabetes on connexin43 mRNA and protein expression in ventricular muscle.

Abnormal QT prolongation with the associated arrhythmias is a significant predictor of mortality in diabetic patients. Gap junctional intercellular communication allows electrical coupling between heart muscle cells. The effects of streptozotocin (STZ)-induced diabetes mellitus on the expression and distribution of connexin 43 (Cx43) in ventricular muscle have been investigated. Cx43 mRNA expression was measured in ventricular muscle by quantitative PCR. The distribution of total Cx43, phosphorylated Cx43 (at serine 368) and non-phosphorylated Cx43 was measured in ventricular myocytes and ventricular muscle by immunocytochemistry and confocal microscopy. There was no significant difference in Cx43 mRNA between diabetic rat ventricle and controls. Total and phosphorylated Cx43 were significantly increased in ventricular myocytes and ventricular muscle and dephosphorylated Cx43 was not significantly altered in ventricular muscle from diabetic rat hearts compared to controls. Disturbances in gap junctional intercellular communication, which in turn may be attributed to alterations in balance between total, phosphorylated and dephosporylated Cx43, might partly underlie prolongation of QRS and QT intervals in diabetic heart.

P2 purinergic receptor mRNA in rat and human sinoatrial node and other heart regions.

It is known that adenosine 5′-triphosphate (ATP) is a cotransmitter in the heart. Additionally, ATP is released from ischemic and hypoxic myocytes. Therefore, cardiac-derived sources of ATP have the potential to modify cardiac function. ATP activates P2X1–7 and P2Y1–14 receptors; however, the presence of P2X and P2Y receptor subtypes in strategic cardiac locations such as the sinoatrial node has not been determined. An understanding of P2X and P2Y receptor localization would facilitate investigation of purine receptor function in the heart. Therefore, we used quantitative PCR and in situ hybridization to measure the expression of mRNA of all known purine receptors in rat left ventricle, right atrium and sinoatrial node (SAN), and human right atrium and SAN. Expression of mRNA for all the cloned P2 receptors was observed in the ventricles, atria, and SAN of the rat. However, their abundance varied in different regions of the heart. P2X5 was the most abundant of the P2X receptors in all three regions of the rat heart. In rat left ventricle, P2Y1, P2Y2, and P2Y14 mRNA levels were highest for P2Y receptors, while in right atrium and SAN, P2Y2 and P2Y14 levels were highest, respectively. We extended these studies to investigate P2X4 receptor mRNA in heart from rats with coronary artery ligation-induced heart failure. P2X4 receptor mRNA was upregulated by 93% in SAN (P < 0.05), while a trend towards an increase was also observed in the right atrium and left ventricle (not significant). Thus, P2X4-mediated effects might be modulated in heart failure. mRNA for P2X4–7 and P2Y1,2,4,6,12–14, but not P2X2,3 and P2Y11, was detected in human right atrium and SAN. In addition, mRNA for P2X1 was detected in human SAN but not human right atrium. In human right atrium and SAN, P2X4 and P2X7 mRNA was the highest for P2X receptors. P2Y1 and P2Y2 mRNA were the most abundant for P2Y receptors in the right atrium, while P2Y1, P2Y2, and P2Y14 were the most abundant P2Y receptor subtypes in human SAN. This study shows a widespread distribution of P2 receptor mRNA in rat heart tissues but a more restricted presence and distribution of P2 receptor mRNA in human atrium and SAN. This study provides further direction for the elucidation of P2 receptor modulation of heart rate and contractility.