Prostaglandins, bioassay and inflammation

Abstract

The formation of the British Pharmacological Society coincided almost exactly with a series of ground-breaking studies that ushered in an entirely new field of research--that of lipid mediator pharmacology. For many years following their chemical characterisation, lipids were considered only to be of dietary or structural importance. From the 1930s, all this changed--slowly at first and then more dramatically in the 1970s and 1980s with the emergence of the prostaglandins (PGs), the first intercellular mediators to be clearly derived from lipids, in a dynamic on-demand system. The PGs exhibit a wide range of biological activities that are still being evaluated and their properties underlie the action of one of the world's all-time favourite medicines, aspirin, as well as its more modern congeners. This paper traces the development of the PG field, with particular emphasis on the skillfull utilisation of the twin techniques of bioassay and analytical chemistry by U.K. and Swedish scientists, and the intellectual interplay between them that led to the award of a joint Nobel Prize to the principal researchers in the PG field, half a century after the first discovery of these astonishingly versatile mediators.

Figure 1

Vane's ‘superfusion cascade'. This experimental set-up – or variants of it – was used throughout the 1960–1980s in the group's experiments on PG release and synthesis. Krebs' solution, animal (or sometimes human) blood, was pumped through a warming coil and allowed to superfuse a selection of isolated tissues ‘in cascade'. These tissues were chosen according to the nature of the experiment and the substances to be assayed (see text), and analysis of the tracings allowed a ‘differential' assessment of the hormones and substances in the fluid to be made in real time. After superfusing the tissues, the blood was returned to the animal through a vein. If Krebs' solution was used, say, because the release of substances from a perfused organ was being investigated, then this was taken to waste or retained for further analysis.

Figure 2

The release of substances from the guinea-pig lung induced by anaphylactic shock. The tissues, including the cat terminal ileum (CTI), the rabbit aortic strip (Rba), the chick rectum (CR), the guinea-pig trachea (GPT), the rat stomach strip (RSS) and the rat colon (RC), were superfused with effluent from a sensitised guinea-pig perfused lung in preparation. Egg albumin (EA) was injected through the lungs (i.a.) and caused a massive release of a substance, which could not be accounted for by the presence of histamine or PGE₂, which were also tested directly (DIR) over the tissues. From Vane (1971b), with permission.

Figure 3

The inhibitory effect of aspirin on ‘RCS' release. The perfusate from ovalbumin-sensitised guinea-pig lungs was used to superfuse a guinea-pig trachea (GPT), rabbit aortic strip (RbA), guinea-pig ileum (GPI), rat stomach strip (RSS), rat colon (RC) and chick rectum (CR). When ovalbumin (Ovalb) was injected into the lungs (IA), the release of ‘RCS' could be clearly seen. This was blocked by the infusion of aspirin when given through the lungs though not when given directly over the tissue (DIR). ‘Calibrating' infusions of PGE₂ are given at various points to assess the sensitivity of the tissues. From Piper & Vane (1969), with permission.

Figure 4

Prostaglandin synthesis from arachidonic acid, as it was known by the mid-1970s. At the beginning of the period discussed in this article, the only structures known were the ‘stable' PGs E, F and D. All three can be produced through non-enzymatic breakdown of the reaction intermediates or through specific enzymatic activity, although this was not obvious to early workers. After the isolation of the intermediate endoperoxides termed ‘PGG₂ and PGH₂' by the Karolinska group (and ‘15-hydroperoxy PGR₂ and PGR₂' by the Unilever team), the family expanded considerably. The stable biologically inactive product observed in lungs and platelets originally termed ‘PHD' was renamed ‘Thromboxane B₂' when the highly unstable ‘Thromboxane A₂' was identified and the nature of Vane's ‘RCS' finally established. MDA, malondialdehyde.

Figure 5

‘Disappearance' of PGH₂ when incubated with microsomes from vascular tissue. The isolated tissues were as indicated. Injection into the superfusing fluid of the endoperoxide PGH₂ caused the rabbit aortic strip to contract, but when mixed with various amounts of microsomes from aorta (AM) the response was reduced or abolished. Calibrating injections of various PGs are also shown. From Gryglewski et al. (1976), with permission.

Figure 6

The effect of ‘PGX' on platelet aggregation. Human platelet-rich plasma was aggregated with arachidonic acid (AA) and the effect recorded in a ‘Born' aggregometer. In the presence of ‘PGX' extracts (see text for details), aggregation is completely abolished. However, if the extracts are warmed in aqueous solution for 10 min, they lose their anti-aggregatory activity. The products of the spontaneous decomposition of PGH₂ are without effects on the aggregation response. From Gryglewski et al. (1976), with permission.

Figure 7

The synthesis of prostacyclin, the last of the ‘major' PGs to be discovered. By the mid-1970s it became clear from bioassay and other experiments that there were other routes of endoperoxide metabolism that were not accounted for by the pathways shown in Figure 4. This led to the discovery of the unstable ‘PGI₂' or ‘Prostacyclin', which decayed spontaneously to the inactive 6-keto-PGF_1α end product.