Fig. 5.

Tβ4 −/Y mice possess limited contractile reserve. Mid-ventricular short axis images, at baseline and under dobutamine stress, were acquired at high frame rate. Tβ4 +/Y: A–D; Tβ4 −/Y: E–H. End systolic volume at baseline of Tβ4 −/Y mice (F) is considerably reduced compared with control (+/Y) mice (B; Table A4; n = 4 +/Y, 6 −/Y), limiting the extent to which LV contractility can be further increased under dobutamine stress (Tβ4 +/Y: D; Tβ4 −/Y: H). Representative volume curves for +/Y (I) and −/Y mice (J) demonstrate reduced contractile reserve (characteristic leftward shift, as observed in +/Y) in −/Y mice upon β-adrenergic stimulation (shown for n = 2 each genotype).