Introduction

I came to work at University College, London, in the autumn of 1973. Andrew Huxley – “Prof” as I soon learned to call him – and I had originally discussed doing tension-transient experiments on slow-tonic fibres, which I had worked on previously. It turned out that the equipment was heavily occupied by other experiments during that autumn, so we decided instead to follow up an old observation by Caspersson and Thorell (1942) that adenine nucleotides appear to be compartmentalised in muscle fibres and might change their distribution upon stimulation. The idea was typical of Prof's sharp eye for older, potentially important observations, and we started to design a setup for the experiments with high hopes. I learned an invaluable amount of basic optics and microscopy during this period, and also that fibres are very easily damaged by intense, short-wavelength, ultraviolet (UV) light. In the end, the results were disappointing: we found no clear evidence of compartmentalisation and certainly no stimulus-related changes in the UV absorbance pattern (Lännergren, 1977).

The negative result meant that there was little incentive to pursue this line. Thus, back in Stockholm I continued my work on contractile properties of various fibre types in Xenopus, now armed with a much better understanding of cross-bridge kinetics after many illuminating sessions with Prof, Bob Simmons, Lincoln Ford, and Yale Goldman. The realisation that there is a broad range of shortening velocities among Xenopus fibres then led us to attempt to sort out the various isomyosins in this species, the results of which are summarised below.