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

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.