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The primary pacemaker area is located at the site with lowest percentage of myofilaments and the highest rate of diastolic depolarization in rabbit, guinea-pig, cat and pig. All investigated sinuatrial nodes contained large amounts, 45% or more, of collagen. There was, however, substantially more collagen in the sinuatrial nodes of the cat and the pig than in the rabbit and the guinea-pig. This had, however, no consequences for the sinuatrial conduction time and the regularity of the beat-to-beat cycle length in the different species, because the rabbit and cat had comparable sinuatrial conduction times, although their nodal collagen content was very different and the beat-to-beat cycle length showed a comparable variability in the different species. We conclude that extensive proliferation of collagen and fibroblasts does not necessarily impair intercellular coupling in the sinuatrial node. [ABSTRACT FROM PUBLISHER]

Three different bradycardic agents, alinidine, AQ-A39 and UL-FS49 increase the intrinsic cycle length of the isolated SA node preparation of the rabbit. This increase is mainly caused by a decrease in rate of diastolic depolarization. One of these agents, alinidine, was used to study the underlying ionic mechanism of the decrease in the diastolic depolarization rate in isolated cells and small cell clusters of the rabbit SA node. In these preparations alinidine slowed down the rate of spontaneous activity at higher concentrations (80 μM). At lower concentrations (10 μM) the decrease in rate of spontaneous activity was variable, but injection of a hyperpolarizing current slowed the spontaneous rate more in the presence of alinidine, indicating an increase in membrane resistance. In voltage clamp experiments we found that the main effect of alinidine was a block of the hyperpolarization activated current ij. The block was potential dependent and was maximal in the potential range in which diastolic depolarization occurs.These results are discussed in relation to previous findings of others. [ABSTRACT FROM PUBLISHER]