ryanodine and Hypertrophy

Transgenic mice with cardiac-specific overexpression of active Akt (TG) not only exhibit hypertrophy but also show enhanced left ventricular (LV) function. In 3-4-month-old TG, heart/body weight was increased by 60% and LV ejection fraction was elevated (84 +/- 2%, p < 0.01) compared with nontransgenic littermates (wild type (WT)) (73 +/- 1%). An increase in isolated ventricular myocyte contractile function (% contraction) in TG compared with WT (6.1 +/- 0.2 versus 3.5 +/- 0.2%, p < 0.01) was associated with increased Fura-2 Ca2+ transients (396 +/- 50 versus 250 +/- 24 nmol/liter, p < 0.05). The rate of relaxation (+dL/dt) was also enhanced in TG (214 +/- 15 versus 98 +/- 18 microm/s, p < 0.01). L-type Ca2+ current (ICa) density was increased in TG compared with WT (-9.0 +/- 0.3 versus 7.2 +/- 0.3 pA/pF, p < 0.01). Sarcoplasmic reticulum Ca2+ ATPase 2a (SERCA2a) protein levels were increased (p < 0.05) by 6.6-fold in TG, which could be recapitulated in vitro by adenovirus-mediated overexpression of Akt in cultured adult ventricular myocytes. Conversely, inhibiting SERCA with either ryanodine or thapsigargin affected myocyte contraction and relaxation and Ca2+ channel kinetics more in TG than in WT. Thus, myocytes from mice with overexpressed Akt demonstrated enhanced contractility and relaxation, Fura-2 Ca2+ transients, and Ca2+ channel currents. Furthermore, increased protein expression of SERCA2a plays an important role in mediating enhanced LV function by Akt. Up-regulation of SERCA2a expression and enhanced LV myocyte contraction and relaxation in Akt-induced hypertrophy is opposite to the down-regulation of SERCA2a and reduced contractile function observed in many other forms of LV hypertrophy.

Topics: Adenoviridae; Alkaline Phosphatase; Animals; Blotting, Western; Body Weight; Calcium; Calcium-Transporting ATPases; Calsequestrin; Dose-Response Relationship, Drug; Down-Regulation; Echocardiography; Electrophysiology; Enzyme Inhibitors; Fura-2; Heart Ventricles; Hypertrophy; Inhibitory Concentration 50; Kinetics; Lysophospholipase; Mice; Mice, Transgenic; Muscle Cells; Organ Size; Protein Serine-Threonine Kinases; Proto-Oncogene Proteins; Proto-Oncogene Proteins c-akt; Rats; Reverse Transcriptase Polymerase Chain Reaction; Ryanodine; Ryanodine Receptor Calcium Release Channel; Sarcoplasmic Reticulum Calcium-Transporting ATPases; Thapsigargin; Time Factors; Transfection; Transgenes; Up-Regulation

1. Partial outlet of obstruction of the rabbit urinary bladder results in a significant increase in the ability of ryanodine to inhibit field stimulated contraction. The current study determined the effect of outlet obstruction on ryanodine binding to microsomal membranes. 2. The results demonstrate: (i) Ryanodine binding is linear showing one set of ryanodine binding sites (Bmax = 28 +/- 3.5 fmol/mg protein; Kd = 4.2 nM). (ii) Total ryanodine binding was slightly increased at 1 and 3 day following obstruction. (iii) Binding increased approx 4-fold at 5 and 7 days post obstruction. (iv) No change in the Kds were noted at any time period. 3. The results of these studies indicate that smooth muscle hypertrophy secondary to partial outlet obstruction induces a marked increase in the role of intracellular calcium in the mediation of the contractile response to field stimulation.

Topics: Animals; Calcium; Hypertrophy; In Vitro Techniques; Intracellular Membranes; Linear Models; Male; Microsomes; Muscle Contraction; Rabbits; Ryanodine; Urinary Bladder Neck Obstruction