Haruo Honjo

Cellular Mechanisms of Sinoatrial Activity

The rhythmic beating of the heart is the result of action potentials initiated in a small specialized tissue, the sinoatrial (SA) node. With a wealth of recent information on the cellular electrophysiology, molecular biology, and microscopic morphology of the SA node, the machinery underlying its robust pacemaking during a variety of physiologic and pathologic conditions is beginning to be fully understood. This chapter discusses (1) the ionic events (and their molecular basis) responsible for the spontaneous activity in SA node cells, and (2) the sophisticated organization of different types of SA node cells within the SA node as a whole to make a robust and dependable pacemaker for the heart.

The Effect of β-adrenargic Agonists on Ca^2+ Sensitivity in Tracheal Smooth Muscle

To determine whether a reduction in the sensitivity to Ca 2 + is involved in relaxation by β-adrenergic receptor agonists, we examined correlation between force generation and intracellular Ca 2 + concentration in the inhibitory effects of isoproterenol (ISO) on methacholine (MCh)-induced contraction. Fura-2 was loaded in tracheal smooth muscle of guinea pigs and tension and fluorescence ratio F 3 4 0 /F 3 8 0 as an index of intracellular Ca 2 + concentration were recorded simultaneously. Addition of 0.3 μM ISO caused an inhibition in 1 μM MCh-induced contraction and a reduction in intracellular Ca 2 + concentration. The reduction in intracellular Ca 2 + concentration was, however, much smaller than that of tension. In conclusion, Ca 2 + sensitivity in addition to Ca 2 + influx plays an important role in β-adrenergic action in airway smooth muscle.

Limited Association of Connexin43 and ZO-1 in the Intercalated Disks of Adult Rat Ventricular Myocardium

Recent biochemical studies have suggested that zonula occludens-1 (ZO-1), a PDZ domain-containing protein may play an important role in targeting the major gap junction protein, Cx43 to the intercalated disk region of cardiac muscle. Information for in vivo is, however, still limited. We investigated the distribution of Cx43 and ZO-1 proteins in the ventricular myocardium of adult rats using high-resolution analysis with a confocal microscopy. Immunolabeling spots for Cx43 and ZO-1 were largely confined to the intercalated disk regions of ventricular muscles. ZO-1 was also localized mainly in the intercalated disk region. These spots were a marked difference in the positions, indicating low levels of colocalization of Cx43 and ZO-1 immunolabled spots in the intercalated disks. These results suggest that ZO-1 does not play an important role in intercalated disk-targeting of Cx43 molecules in the normal ventricular myocardium.

Role of Ca^ Release from Sarcoplasmic Reticulum in Pacemaker Activity of the Sinoatrial Node

Recent studies using confocal microscopy combined with patch clamping on single sinoatrial (SA) node pacemaker cells suggest that Ca 2 + release from the sarcoplasmic reticulum (SR) during diastole may play a prominent role in the late phase of pacemaker depolarization. The present study was designed to test this hypothesis in the intact SA node. We investigated the effects of a high concentration of ryanodine, which is known to disable SR Ca 2 + release, on spontaneous activity of isolated rabbit right atrium including the whole SA node by using an extracellular potential mapping technique. Inhibition of SR Ca 2 + release by 30 μM ryanodine caused only a moderate reduction of the spontaneous firing rate (by 20.0′2.8 %, n=4) of the intact SA node. This observation is inconsistent with previous data obtained from single pacemaker cells. Physiological significance of SR Ca 2 + release in the regulation of SA node pacemaker activity is still unsettled.

Dephosphorylation of Connexin43 Associated with Ventricular Hypertrophy

Altered expression and distribution of gap junctions in hypertrophied hearts may provide a potential substrate for abnormal conduction. We investigated changes of phosphorylation state of connexin43 (Cx43) in hypertrophied rat right ventricles secondary to pulmonary hypertension induced by monocrotaline (MCT) using western blot analysis. In normal right ventricular myocardium, most of Cx43 was phosphorylated. In hypertrophied ventricles of MCT-treated rats, the non-phosphorylated isoform of Cx43 increased and the phosphorylated isoform of Cx43 decreased with no significant a change in the total amount of Cx43 protein. These results suggest that dephosphorylation of Cx43 in MCT-induced right ventricular hypertrophy may be involved in gap junction disorganization.