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An X-Ray Diffraction Study on Mouse Cardiac Cross-Bridge Function In Vivo: Effects of Adrenergic ß-Stimulation

Ryuji Toh ^*, Masakazu Shinohara ^*, Tomofumi Takaya ^*, Tomoya Yamashita ^*, Shigeru Masuda ^*, Seinosuke Kawashima ^*, Mitsuhiro Yokoyama ^* and Naoto Yagi 

^* Division of Cardiovascular and Respiratory Medicine, Department of Internal Medicine, Kobe University Graduate School of Medicine, Kobe 650-0017, Japan; and SPring-8/JASRI, Sayo, Hyogo 679-5198, Japan

Correspondence: Address reprint requests to N. Yagi, SPring-8/JASRI, Kouto, Sayo, Hyogo 679-5198, Japan. Tel.: 81-791-58-0908; Fax: 81-791-58-0830; E-mail: yagi{at}spring8.or.jp.

To investigate how ß-stimulation affects the contractility of cardiac muscle, x-ray diffraction from cardiac muscle in the left ventricular free wall of a mouse heart was recorded in vivo. To our knowledge, this is the first x-ray diffraction study on a heart in a living body. After the R wave in electrocardiograms, the ratio of the intensities of the equatorial (1,0) and (1,1) reflections decreased for 50 ms from a diastolic value of 2.1 to a minimum of 0.8, and then recovered. The spacing of the (1,0) lattice planes increased for 90 ms from a diastolic value of 37.2 nm to a maximum of 39.1 nm, and then returned to the diastolic level, corresponding to 10% stretch of sarcomere. Stimulation of ß-adrenergic receptor by dobutamine (20 µg/kg/min) accelerated both the decrease in the intensity ratio, which reached a smaller systolic value, and the increase in the lattice spacing. However, the intensity ratio and spacing at the end-diastole were unchanged. The recovery of the lattice spacing during relaxation was also accelerated. The mass transfer to the thin filaments at systole in a ß-stimulated heart was close to the peak value in twitch of frog skeletal muscle at 4°C, showing that the majority of cross-bridges have been recruited with few in reserve.

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