A new approach for measuring real-time calcium pumping and SR function in muscle fibers

Small Ca2+ releases enable hour-long high-frequency contractions in midshipman swimbladder muscle

The swimbladder muscle of the Pacific midshipman fish contracts up to 360,000 times in an hour while generating mating calls. Using experimental measurements and computational modeling, Nelson et al. reveal the Ca²⁺ handling that permits these superfast muscle fibers to sustain high-frequency calling.

Type I males of the Pacific midshipman fish (Porichthys notatus) vibrate their swimbladder to generate mating calls, or “hums,” that attract females to their nests. In contrast to the intermittent calls produced by male Atlantic toadfish (Opsanus tau), which occur with a duty cycle (calling time divided by total time) of only 3–8%, midshipman can call continuously for up to an hour. With 100% duty cycles and frequencies of 50–100 Hz (15°C), the superfast muscle fibers that surround the midshipman swimbladder may contract and relax as many as 360,000 times in 1 h. The energy for this activity is supported by a large volume of densely packed mitochondria that are found in the peripheral and central regions of the fiber. The remaining fiber cross section contains contractile filaments and a well-developed network of sarcoplasmic reticulum (SR) and triadic junctions. Here, to understand quantitatively how Ca²⁺ is managed by midshipman fibers during calling, we measure (a) the Ca²⁺ pumping-versus-pCa and force-versus-pCa relations in skinned fiber bundles and (b) changes in myoplasmic free [Ca²⁺] (Δ[Ca²⁺]) during stimulated activity of individual fibers microinjected with the Ca²⁺ indicators Mag-fluo-4 and Fluo-4. As in toadfish, the force–pCa relation in midshipman is strongly right-shifted relative to the Ca²⁺ pumping–pCa relation, and contractile activity is controlled in a synchronous, not asynchronous, fashion during electrical stimulation. SR Ca²⁺ release per action potential is, however, approximately eightfold smaller in midshipman than in toadfish. Midshipman fibers have a larger time-averaged free [Ca²⁺] during activity than toadfish fibers, which permits faster Ca²⁺ pumping because the Ca²⁺ pumps work closer to their maximum rate. Even with midshipman’s sustained release and pumping of Ca²⁺, however, the Ca²⁺ energy cost of calling (per kilogram wet weight) is less than twofold more in midshipman than in toadfish.

Cross-bridge blocker BTS permits direct measurement of SR Ca2+ pump ATP utilization in toadfish swimbladder muscle fibers

Because the major processes involved in muscle contraction require rapid utilization of ATP, measurement of ATP utilization can provide important insights into the mechanisms of contraction. It is necessary, however, to differentiate between the contribution made by cross-bridges and that of the sarcoplasmic reticulum (SR) Ca2+ pumps. Specific and potent SR Ca2+ pump blockers have been used in skinned fibers to permit direct measurement of cross-bridge ATP utilization. Up to now, there was no analogous cross-bridge blocker. Recently, N-benzyl-p-toluene sulfonamide (BTS) was found to suppress force generation at micromolar concentrations. We tested whether BTS could be used to block cross-bridge ATP utilization, thereby permitting direct measurement of SR Ca2+ pump ATP utilization in saponin-skinned fibers. At 25 microM, BTS virtually eliminates force and cross-bridge ATP utilization (both <4% of control value). By taking advantage of the toadfish swimbladder muscle's unique right shift in its force-Ca2+ concentration ([Ca2+]) relationship, we measured SR Ca2+ pump ATP utilization in the presence and absence of BTS. At 25 microM, BTS had no effect on SR pump ATP utilization. Hence, we used BTS to make some of the first direct measurements of ATP utilization of intact SR over a physiological range of [Ca2+]at 15 degrees C. Curve fits to SR Ca2+ pump ATP utilization vs. pCa indicate that they have much lower Hill coefficients (1.49) than that describing cross-bridge force generation vs. pCa (approximately 5). Furthermore, we found that BTS also effectively eliminates force generation in bundles of intact swimbladder muscle, suggesting that it will be an important tool for studying integrated SR function during normal motor behavior.

Superfast contractions without superfast energetics: ATP usage by SR-Ca2+ pumps and crossbridges in toadfish swimbladder muscle

1. The rate at which an isometrically contracting muscle uses energy is thought to be proportional to its twitch speed. In both slow and fast muscles, however, a constant proportion (25-40 %) of the total energy has been found to be used by SR-Ca2+ pumps and the remainder by crossbridges. We examined whether SR-Ca2+ pumps account for a larger proportion of the energy in the fastest vertebrate muscle known (the toadfish swimbladder), and whether the swimbladder muscle utilizes energy at the superfast rate one would predict from its mechanics. 2. The ATP utilization rates of the SR-Ca2+ pumps and crossbridges were measured using a coupled assay system on fibres skinned with saponin. Surprisingly, despite its superfast twitch speed, the ATP utilization rate of swimbladder was no higher than that of much slower fast-twitch amphibian muscles. 3. The swimbladder achieves tremendous twitch speeds with a modest steady-state ATP utilization rate by employing two mechanisms: having a small number of attached crossbridges and probably utilizing intracellular Ca2+ buffers (parvalbumin) to spread out the time over which Ca2+ pumping can occur. 4. Finally, although the total ATP utilization rate was not as rapid as expected, the relative proportions used by SR-Ca2+ pumps and the crossbridges were similar to other muscles.