Differences In The Volume Of Stress Platelets And Vincristine Stimulated Platelets

The mechanisms by which thrombocytoses are produced are not understood. This study examines the size of platelets in thrombocytoses produced by two different stimuli.

Rats were given Vincristine (0.1 mg/kg intravenously) which produced an increase in platelet count from 1.0 to 1.5 × 10⁹/ml on day 5 without a preceding thrombocytopenia. The volume of platelets from an aliquot of the whole platelet population was measured daily using a Coulter ZF counter coupled to a 2000 channel multichannel analyser.

Platelet volume was found to be log Gaussian with a mean modal volume of 2.44 f1. in a control group of rats (n=8). There was no significant difference between platelet volumes in the Vincristine treated group and the control group on any day.

In a second experiment platelet antiserum (PAS) or saline (control) was injected into groups of five rats. PAS produced a platelet count of 8% relative to the control group on day 1. Platelet modal volume was measured daily and in the control group did not significantly vary from 2.28±0.14 f1. In the PAS group on day 1 platelet volume was 4.17 ±0.70 f1, an increase of 83%. By day 6 the platelet count was 1.6 × 10⁹/ml. and modal volume was 2.42 ± 0.08 f1. Platelet volume and count returned to control group levels by days 8 and 9 respectively.

Bone marrow microdensitometry showed a change in megakaryocyte ploidy number after injection of PAS.

The rise in platelet counts produced by vincristine and after thrombocytopenia (stress platelets) are probably produced by different mechanisms.