Mechanisms underlying the frequency dependence of contraction and [Ca super(2+)] sub(i) transients in mouse ventricular myocytes

In most mammalian species force of contraction of cardiac muscle increases with increasing rate of stimulation, i.e. a positive force-frequency relationship. In single mouse ventricular cells, both positive and negative relationships have been described and little is known about the underlying mechanisms. We studied enzymatically isolated single ventricular mouse myocytes, at 30 degree C. During field stimulation, amplitude of unloaded cell shortening increased with increasing frequency of stimulation (0.04 plus or minus 0.01 Delta L/L sub(0) at 1 Hz to 0.07 plus or minus 0.01 Delta L/L sub(0) at 4 Hz, n = 12, P < 0.05). During whole cell voltage clamp with 50 mu M [K5-fluo-3] sub(pip), both peak and baseline [Ca super(2+)] sub(i) increased at higher stimulation frequencies, but the net Delta [Ca super(2+)] sub(i) increased only modestly from 1.59 plus or minus 0.08 Delta F/F sub(0) at 1 Hz, to 1.71 plus or minus 0.11 Delta F/F sub(0) at 4 Hz (n = 17, P < 0.05). When a 1 s pause was interposed during stimulation at 2 and 4 Hz, [Ca super(2+)] sub(i) transients were significantly larger (at 4 Hz, peak F/F sub(0) increased by 78 plus or minus 2%, n = 5). SR Ca super(2+) content assessed during caffeine application, significantly increased from 91 plus or minus 24 mu mol l super(-1) at 1 Hz to 173 plus or minus 20 mu mol l super(-1) at 4 Hz (n = 5, P < 0.05). Peak I sub(Ca,L) decreased at higher frequencies (by 28 plus or minus 6% at 2 Hz, and 45 plus or minus 8% at 4 Hz), due to slow recovery from inactivation. This loss of I sub(Ca,L) resulted in reduced fractional release. Thus, in mouse ventricular myocytes the [Ca super(2+)] sub(i)-frequency response depends on a balance between the increase in SR content and the loss of trigger I sub(Ca,L). Small changes in this balance may contribute to variability in frequency-dependent behaviour. In addition, there may be a regulation of the contractile response downstream of [Ca super(2+)] sub(i).