The thromboxane mimetic, U‐46619, induces the aggregation of fish thrombocytes

Abstract

Fish blood contains nucleated cells termed thrombocytes which are thought to be functionally analogous to mammalian platelets. The present study was undertaken to assess the aggregatory response of peripheral blood thrombocytes from the rainbow trout, Oncorhynchus mykiss. Incubation of Percoll density gradient purified thrombocytes with the thromboxane A2 mimetic, U-46619 (0.03-10 microM), alone or in the presence of human or trout fibrinogen, elicited a dose-dependent aggregatory response. A greater amount of aggregation was observed in the presence of fish, rather than human, fibrinogen (e.g. c 55% compared with 15% maximal aggregation respectively). This response was inhibited by pre-incubation of thrombocytes with the specific thromboxane A2 receptor antagonist, GR32191 (0.01-10 microM) with an IC50 value of 5.7 microM. Ultrastructural and aggregometry studies of thrombocyte aggregation at various time periods (1-12 min) after incubation with U-46619 (0.5 microM) revealed clear differences between the amount and dynamics of thrombocyte clumping in the presence of human, compared with trout, fibrinogen (final concentration 400 micrograms ml-1). Thrombocytes rapidly underwent shape change and aggregation after only 1 min incubation without the initial involvement of any other cell types. The maximum degree of aggregation was achieved after 4-8 min with larger aggregates formed in the presence of trout compared than human fibrinogen. Ultrastructurally, the thrombocytes in these aggregates displayed a number of changes compared with non-stimulated cells, including increased pseudopodial activity, a more pronounced canalicular system and condensation of the nuclear heterochromatin. After 12 min incubation the clumps of thrombocytes showed progressive disaggregatory behaviour, with some cells reverting back to their normal in vivo appearance. Neutrophilic granulocytes, present as contaminants in the thrombocyte suspensions, were occasionally found attached to the thrombocyte aggregates, perhaps suggesting a specific interaction similar to that occurring in mammals. Finally, and also of some significance, was the finding that trout thrombocytes exhibit phagocytic activity in vitro towards cell debris and contaminating bacteria, indicating that this cell type may possess both haemostatic and immunological capacities.

Haematology and leucocyte enzyme cytochemistry of a threatened yellow catfish Horabagrus brachysoma (Gunther 1864)

“…The phagocytic activity of fish thrombocytes has been reported earlier (Hill and Rowley 1996;Slierendrecht et al 1995;Burrows et al 2001;Stosik et al (2001;Kolmam et al (2003). Therefore, the thrombocytes of H. brachysoma can act as blood phagocytes that form one of the protective barriers and may be considered true digesting cells in fish because of the presence of PER and specific esterase enzymes, although more investigations are needed to prove this.…”

A morphological and cytochemical study of erythrocytes, thrombocytes and leukocytes in four freshwater teleosts

“…Thrombocytes, in addition to participating in defence through phagocytosis (Hill & Rowley, 1996; Burrows et al , 2001; Tavares‐Dias & Moraes, 2004), are also required for haemostatic plug or fibrin clot (Hill & Rowley, 1996) and they support such energy‐requiring functions, as clot retraction, even though blood glucose may be less readily available (Hayhoe & Quaglino, 1994). Hence, it is necessary to maintain large glucose reserves (Hayhoe & Quaglino, 1994) in the form of glycogen granules (Ueda et al , 2001; Tavares‐Dias & Moraes, 2004; Tavares‐Dias et al , 2004).…”

Ultrastructure of novel thrombocytes in the dog snapper Lutjanus jocu

“…Previous studies suggest that morphological variability between TC occurs in response to specific physiological conditions (Hill & Rowley, ). Phagocytic activity is broadly recognized as a function of the leukocytes (Silva et al , ) but it has also been suggested that TCs have a role in this immune response (Hill & Rowley, ; Burrows et al , ; Tavares‐Dias et al , ; Shigdar et al , ; Silva et al , ). In this study, the fact that the TC displayed ultrastructural showing vacuolar content being released through canaliculi, suggests that the content may be a product of phagocytic activity (Hill & Rowley, ).…”

“…Phagocytic activity is broadly recognized as a function of the leukocytes (Silva et al , ) but it has also been suggested that TCs have a role in this immune response (Hill & Rowley, ; Burrows et al , ; Tavares‐Dias et al , ; Shigdar et al , ; Silva et al , ). In this study, the fact that the TC displayed ultrastructural showing vacuolar content being released through canaliculi, suggests that the content may be a product of phagocytic activity (Hill & Rowley, ). These observations may support the hypothesis that TCs possess an active role in phagocytic activity (Burrows et al , ; Tavares‐Dias et al , ; Zexia et al , ; Silva et al , ).…”

“…Studies have highlighted the variability of morphological characteristics among TCs (Tierney et al , ; Rough et al , ; Tavares‐Dias, ; Zexia et al , ; Silva et al , ) with the occurrence of cytoplasmic extensions, vesicles, vacuoles and microtubular systems suggesting phagocytic activity (Zinkl et al , ; Burrows et al , ; Tripathi et al , ; Rough et al , ; Tavares‐Dias, , Shigdar et al , , ; Tavares‐Dias & Moraes, ; Tavares‐Dias et al , ; Zexia et al , ; Silva et al , ). Most of these studies are based on light microscopic observations and cytochemical staining with only a few reporting the ultrastructural features (Daimon & Uchida, ; Hill & Rowley, ; Tripathi et al , ; Tavares‐Dias et al , ; Zexia et al , ; Dezfuli et al , ; Silva et al , ).…”