Having worked on bird salt glands in Hong Kong I arrived at Babraham in October 1968 to work on the mammary gland and the mechanism of milk secretion—a very different gland and a very different fluid being produced.
During that summer Jim Linzell had attended a satellite symposium of the Washington Physiological Congress on exocrine glands and had heard Mabel Hokin give a talk on the bird salt gland. He told me about this and since a similar approach might provide information on the mechanism of secretion of the aqueous phase of milk, I set about doing similar studies on the mammary gland. However, when I read Mabel Hokin’s work in greater detail I realised there were some anomalies. I will not go into great detail but the concentrations of sodium, potassium and chloride ions in salt-gland slices appeared to be much higher than those reported by others. This mattered since by calculating the concentrations inside the cells from a knowledge of the size and composition of the extracellular space, she concluded that the high concentration difference of sodium chloride was established between blood and the inside of the cell, not between the cell and the lumen of the gland, as previous evidence had suggested. I, egged on by Jim Linzell and Richard Keynes FRS who was then Director of the institute and gathering information for his masterly review of ion transport mechanisms across membranes in different organs, thought it wise to repeat the Hokin experiments.
I was unable to get the same results as Mabel Hokin. The concentrations inside the cell appeared similar to other non-nervous tissues, implying that the high concentration gradient for salt is set up between the cell and the lumen of the gland, the later leading to the duct system and the flow of salt-rich fluid to the outside world.
The difference between mine and her results rested on the crude concentrations of sodium, potassium and chloride in the tissue, before any calculation of concentrations inside the cells. I did all sorts of studies with other tissues to see if the methods I was using gave results different from those reported by others. They didn’t. Were the salts being liberated from the tissue fully before analysis? Yes. I did all the standard tricks of analytical chemistry to check that when I added a known amount to the samples I recovered that same amount during analysis. Was I making a stupid arithmetical error in calculating the concentrations? I persuaded three colleagues to work through the calculations independently; they got the same answers.
I was—and still am—at a lost to explain how Hokin found such high concentrations of all three ions of interest. However, it is interesting that her concentrations in tissue were on average 1.94 times higher; double, in other words. Richard Keynes and I concluded privately that somewhere along the line Mabel Hokin had got her sums wrong by a factor of 2. In the days before spreadsheets and even before electronic calculators came into use, the chances of systemic error were increased. Keynes himself hogged the first computer (Olivetti Programma 101) to be installed at Babraham to recalculate some figures on ion movements across an epithelium or membrane originally worked on by a co-worker. ‘Well’, he said after his marathon session, ‘they were out by a factor of 10—100 in one direction and 1000 in the other’.
After the publication of my results, I sent Mabel Hokin an offprint but I received no reply. Her ideas on how the salt gland might work were not heard of again. But why was she working on the salt gland at all?
Mabel Hokin had a very interesting personal history as well as a key role in the uncovering of a major biochemical mechanism by which signals are passed within cells, even though she did not realise it at the time.
Sir Hans (then Professor) Krebs had an enviable reputation when he was at the University of Sheffield (before his defection to Oxford) of employing school leavers as technicians and then seeing the good ones through a university degree course and postgraduate research for a PhD. A number stayed with him as fully-fledged scientists for decades, or nested successfully elsewhere. Mabel Hokin was one of those school leavers.
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| Mabel Hokin the 1980s Photograph used on Wikipedia |
According to a potted biography on Wikipedia. Mabel Neaverson was born in Sheffield in 1924 to working-lass parents. Throughout her life she suffered from an autoimmune disease. After spending 1942-43 in the Land Army she joined the Krebs Cell Metabolism Unit as a technician in 1943. In 1946 she became an undergraduate student (funded at least in part by Sheffield Education Committee) and in the same year married her first husband, the actor, playwright, critic and university lecturer Dennis Davison* (1923-1994); he was also at the University and they met through her interest in costume design and the theatre. Her political activities at this time in the socialist society at university (Chairman, 1947-48) and as a member, with Davison, of the Communist Party of Great Britain were to have consequences a few years later.
After graduation in 1949 (II(i) Hons. Physiology) Krebs suggested she should continue as a research student. Her supervisor was Quentin Gibson (1918-2011, FRS 1969) then in Physiology—apparently unhappily—at Sheffield. Her PhD, funded by the Medical Research Council) was awarded in 1952. I have only found one publication from that period which shows she was working on acetate metabolism in pigeon breast muscle.
As she started her PhD, Lowell Edward Hokin (1924-2018†) was arriving in Sheffield from the U.S.A., accompanied by his first wife, to work with Krebs. Then, as the Wikipedia entry reads: ‘In a short time, Mabel met Krebs' graduate student Lowell Hokin, and the two began a romantic as well as professional relationship’. Divorces were obtained; they were married in Canada in December 1952 where they had arrived from Sheffield in April.
Rather than McGill University in Montreal, they had hoped to move from Sheffield to the U.S.A. but Mabel’s entry was completely blocked during the McCarthy era because of her communist party membership.
In experiments started at Sheffield and completed in Montreal they found that when pancreatic slices were induced to secrete digestive enzymes by cholinergic neurotransmitters, radioactive phosphorus was incorporated at a greater rate into a chemical fraction of the cell they thought was RNA. However, the radioactive label was not in RNA but in the phospholipids of the cell membranes. Until then the phospholipids were ‘regarded as inert structural components of membranes’. Later, they showed the particular phospholipids that were labelled. The Hokins were the first people to demonstrate lipid turnover caused by the stimulation of receptors on the cell. The phenomenon became known as the ‘PI effect’.
The significance of the phenomenon they had discovered was not uncovered for some years. And this is where their own work took a turn in completely in the wrong direction. Because the PI effect was apparent in other organs when stimulated to secrete, they thought that the phospholipids must be involved in transporting substances across cell membranes. Because it occurred in organs like the newly-discovered salt gland which do not secrete enzymes they argued that their phenomenon must constitute the pump which needs energy to carry sodium across cell membranes. They erected a scheme called the ‘phosphatidic acid cycle’ which, they argued, carried sodium from one side of a membrane to the other. Their hypothesis received a great deal of publicity at the time with papers in Nature and Scientific American. But then their whole scheme fell apart. All sorts of evidence accumulated to show that their phosphatidic acid cycle did not fit the bill as a transporter of ions. The world moved on. But that is how Mabel Hokin came to work on salt glands. The announcement of the discovery of salt glands by Knut Schmidt-Nielsen in 1957 coincided with the Hokins seeking an organ that secreted sodium at a high rate and that was stimulated by cholinergic nerves.
Eventually, the ban on Mabel’s admission to the U.S.A. was lifted and the Hokins had moved from Montreal to the University of Wisconsin at Madison in 1957. This is not the place to describe their later work other than to point out that they were divorced in 1971, shortly after the work on the phosphatidic acid cycle as a transport mechanism was ended and the notion shot down.
The ‘PI effect’, however, took off, as described by Bob Michell FRS in his obituary of Mabel written for Biochemist shortly after her death in 2003:
Mabel and her scientific partner and ex-husband Lowell Hokin were amongst the few scientists who have initiated a new scientific field. Their work on stimulated phosphoinositide turnover in secretory tissues, most crucially in the 1950s and 1960s, was a slow fuse that finally ignited an explosion of work that made inositol phospholipids into star players in transmembrane signalling and many other cell regulatory processes. The extraordinary versatility of phosphoinositides would have come to light at some time without their work, but it is not clear how, and it would have taken much longer…
It took another decade for the research community to realize that the initiating event of the Hokins’ ‘PI response’ is phospholipase C-catalysed hydrolysis of PtdIns(4,5)P2, which many cell- surface receptors harness as their central signal-transducing event. Later, 3-kinase-catalysed formation of PtdIns(3,4,5)P3 emerged as a second widespread signalling reaction, and a plethora of other roles for phosphorylated derivatives of PtdIns in central cell functions have since been uncovered.
Mabel and Lowell Hokin laid the foundations on which all of these recent discoveries stand…She and Lowell never deduced exactly what their observations meant, largely because they were doing experiments that were ‘ahead of their time’, but those of us who followed could confidently use their beautiful results to develop new interpretations.
Mabel was a gregarious and enthusiastic woman who never lost her North Country bluntness. Even when on crutches after having new hip joints, she would be dancing at a meeting party soon after coming off a long flight…
After I first read that I felt like I had shot Bambi.
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| The only photograph I have found of Mabel and Lowell Hokin at about the time they were working on salt glands. A meeting in March 1966 on the Neural Properties of Biogenuc Amines |
*Davison went to Australia in 1957 eventually becoming Senior Lecturer in English at Monash University in Australia.
†Lowell Hokin died last month (6 September 2018) in Colorado.
Hokin MR. 1967. The Na+, K+ and Cl content of goose salt gland slices and the effects of acetylcholine and ouabain. Journal of General Physiology 50, 2197-2209.
Hokin MR. 1969. Electrolyte transport in the avian salt gland. In, Exocrine Glands. Proceedings of a Satellite Symposium of the XXIV International Congress of Physiological Sciences. Edited by Botelho SY, Brooks FP, Shelley WB, p 73-83. Philadelphia: University of Philadelphia Press.
Keynes RD. 1969. From frog skin to sheep rumen: a survey of transport of salts and water across multicellular structures. Quarterly Reviews of Biophysics 2, 177-281.
Kresge N, Simoni RD, Hill RL. 2005. A role for phosphoinositides in signaling: the work of Mabel R. Hokin and Lowell E. Hokin. Journal of Biological Chemistry 280, e27.
Michell R. 2003. Mabel R. Hokin (1924–2003). Biochemist, December 2003, 62-63.
Peaker M. 1971. Intracellular concentrations of sodium, potassium and chloride in the salt-gland of the domestic goose and their relation to the secretory mechanism. Journal of Physiology 213, 399-410.
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