In diffusely growing plant cells, cortical microtubules play an important role in regulating the direction of cell expansion. Arabidopsis (Arabidopsis thaliana) spiral2 (spr2) mutant is defective in directional cell elongation and exhibits right-handed helical growth in longitudinally expanding organs such as root, hypocotyl, stem, petiole, and petal. The growth of spr2 roots is more sensitive to microtubule-interacting drugs than is wild-type root growth. The SPR2 gene encodes a plant-specific 94-kD protein containing HEAT-repeat motifs that are implicated in protein-protein interaction. When expressed constitutively, SPR2-green fluorescent protein fusion protein complemented the spr2 mutant phenotype and was localized to cortical microtubules as well as other mitotic microtubule arrays in transgenic plants. Recombinant SPR2 protein directly bound to taxol-stabilized microtubules in vitro. Furthermore, SPR2-specific antibody and mass spectrometry identified a tobacco (Nicotiana tabacum) SPR2 homolog in highly purified microtubule-associated protein fractions from tobacco BY-2 cell cultures. These results suggest that SPR2 is a novel microtubule-associated protein and is required for proper microtubule function involved in anisotropic growth.

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N. Mazgalev. Determinants of LV diastolic function during atrial fibrillation: beat-to-beat analysis in acute dog experiments. Am J Physiol Heart Circ Physiol 286: H145-H152, 2004. First published September 4, 2003 10.1152/ ajpheart.00588.2003 diastolic function during atrial fibrillation (AF) remains poorly understood due to the complex interaction of factors and beat-to-beat variability. The purpose of the present study was to elucidate the physiological determinants of beat-to-beat changes in LV diastolic function during AF. The RR intervals preceding a given cardiac beat were measured from the right ventricular electrogram in 12 healthy open-chest mongrel dogs during AF. Doppler echocardiography and LV pressure and volume beat-tobeat analyses were performed. The LV filling time (FT) and early diastolic mitral inflow velocity-time integral (Evti) were measured using the pulsed Doppler method. The LV end-diastolic volume (EDV), peak systolic LV pressure (LVP), minimum value of the first derivative of LV pressure curve (dP/dtmin), and the time constant of LV pressure decay () were evaluated with the use of a conductance catheter for 100 consecutive cardiac cycles. Beat-to-beat analysis revealed a cascade of important causal relations. LV-FT showed a significant positive linear relationship with Evti (r ϭ 0.87). Importantly, there was a significant positive linear relationship between the RR interval and LV-EDV in the same cardiac beat (r ϭ 0.53). Consequently, there was a positive linear relationship between LV-EDV and subsequent peak systolic LVP (r ϭ 0.82). Furthermore, there were significant positive linear and negative curvilinear relationships between peak systolic LVP and dP/dtmin (r ϭ 0.95) and (r ϭ Ϫ0.85), respectively, in the same cardiac beat. In addition, there was a significant negative curvilinear relationship between dP/dt min and (r ϭ Ϫ0.86). We have concluded that the determinants of LV diastolic function in individual beats during AF depend strongly on the peak systolic LVP. This suggests that the major benefit of slower ventricular rate appears related to lengthening of LV filling interval, promoting subsequent higher peak systolic LVP and greater LV relaxation. filling interval; left ventricular pressure ATRIAL FIBRILLATION (AF) has been long recognized as one of the most frequent chronic arrhythmias, affecting 0.9% of the population and characterized by irregular rhythm (6). The rapid ventricular rate adversely affects left ventricular (LV) function as preload and contractility diminish due to shorter filling intervals. Furthermore, the absence of optimal atrial boosterpump function as well as beat-to-beat changes in preload, afterload, and contractility are serious negative hemodynamic consequences of AF. They may cause severe LV systolic dysfunction similar to tachycardia-induced cardiomyopathy, even in the absence of other heart disease (10).It is well recognized that patients with normal sinus rhythm who arrive at the hospital with heart failure are frequently found to have diastolic ra...