At the heart of every muscle are myosin molecules, true molecular motors fuelled by ATP that drive every contraction. But myosin doesn't come in one flavour alone. One of myosin's components, the myosin heavy chain, comes in many forms, each with a unique set of characteristics, which define each muscle fibre type. Small animals, such as mice and rats, tend to produce the faster isoforms, including a form called 2B, while larger animals' muscles contained slower isoforms, such as 2A, 2X and slow. But pig skeletal muscle seemed to deviate from the norm, comprising slow and fast isoforms. Carlo Reggiani in Padova, Italy, was puzzled; were pigs genuinely producing 2B, a myosin heavy chain that had only been found in smaller creatures previously?Reggiani and his multidisciplinary team decided to resolve the problem(p. 1875).
First the team characterised the contractile properties of individual muscle fibres from several different muscle types. Using a force transducing rig, Reggiani and Giuseppe D'Antona measured the velocity of each contraction,and the maximum force generated, before gently extracting the muscle's motor protein, ready for identification. But identifying individual isoforms was easier said then done.
Using antibody staining and analysing the mRNA in muscles, the team attempted to identify the myosin isoforms in each fibre type, with some success, but Reggiani knew there was only one sure-fire way of identifying the isoforms in individual muscle fibres: separating them in a polyacrylamide gel. But seperating two almost identical myosin isoforms on a gel is extremely challenging. After painstakingly manipulating the gel's structure and composition, Luana Toniolo's patience was finally rewarded when she resolved the minute differences between each isoform, and successfully identified the isoforms present in each fibre type.
Sure enough Toniolo discovered fibres that only contained the slower isoforms, slow, 2A or 2X usually found in large animals. But she also found hybrid fibres comprising 2A-2X, and also 2X-2B isoforms in the longissimus dorsi. So some pig skeletal muscles did contain the very fast 2B isoform, but always in hybrid fibres associated with the slower 2X isoform.
Could Reggiani find a pig muscle whose fibres were comprised exclusively of 2B? He turned to his colleagues in the veterinary school for advice. The team knew that muscles made from 2B alone would have specific characteristics, and Marco Patruno and Francesco Mascarello would have the anatomical know-how to identify potential candidates. Patruno and Mascarello suggested that Reggiani test the retractor bulbi muscle in the animal's eye. The team ran through their suite of myosin diagnosing techniques; retractor bulbi muscles contain pure 2B fibres! The team had found the first 2B muscle fibres in a large mammal.
But the team was in for another surprise; pig muscle fibres containing the 2A and 2X isoforms were much faster than 2A and 2X fibres from similarly sized animals. The pig's muscles hadn't scaled up in the same way as other large animals. Why are pig muscles so much faster than other large animals?
Reggiani suspects it's something to do with the pig's destiny. Bred for the diner table, pigs have been selected for muscle size rather than activity. And it has obviously paid off. According to Reggiani, cuts from the longissimus dorsi are particularly tasty.