GUANINE-NUCLEOTIDES AND CA-2+-DEPENDENT LYSOSOMAL SECRETION IN ELECTROPERMEABILIZED HUMAN PLATELETS

Metabolically stable analogues of GTP, e.g. guanosine 5′‐[γ‐thio]trio]triphosphate (GTP[S]) and guanosine 5′‐[β‐γ‐imido]triphosphate (pp[NH]pG), enhance the extent of Ca2+‐dependent secretion of β‐N‐acetylgluco‐saminidase and β‐galactosidase from electropermeabilised human platelets in the presence of less than 5 μM Ca2+.A similar effect is observed on addition either of 1,2‐dioctanoin or of GTP in the presence or absence of thrombin. In the presence of higher Ca2+ concentrations the extent of enhancement of lysosomal secretion declines and little, or no, enhancement is observed at a [Ca2+] of 30–40 μM. Addition of leupeptin or antipain prevents this decrease in lysosomal secretion and enhances the extent of Ca2+‐dependent lysosomal secretion obtained in the presence or absence of guanine nucleotides, thrombin or 1,2‐dioctanoin. The concentration of GTP[S] or pp[NH]pG required to obtain half‐maximal enhancement of lysosomal secretion is dependent on [Ca2+] for secretion of 5‐hydroxytryptamine, β‐N‐acetylglucosaminidase and β‐galactosidase. At two fixed [Ca2+] the median effective concentration (EC50) values for GTP[S] and pp[NH]pG which characterise enhancement of 5‐hydroxytryptamine secretion are significantly different from those characterising enhancement of the secretion of β‐N‐acetylglucosaminidase and β‐galactosidase. In the presence of a saturating concentration of GTP[S] marked 5‐hydroxytryptamine and β‐N‐acetylglucosaminidase secretion is observed at nanomolar [Ca2+] and these responses show little dependence on [Ca2+] over the attainable range. Secretion of β‐N‐acetylglucosaminidase is also induced at nanomolar Ca2+ concentrations by addition of activators of protein kinase C. Guanosine 5′‐[β‐thio]diphosphate inhibits enhancement of β‐N‐acetylglucosaminidase secretion induced by GTP[S] but has no effect on secretion of this enzyme induced by Ca2+ when added alone. Our data provide some support for a model in which addition of metabolically stable guanine nucleotides enhances Ca2+‐dependent platelet lysosomal secretion by activating a guanine‐nucleotide‐binding protein (GE) located close to the exocytotic site. However, not all the data are consistent with this postulate. Copyright © 1990, Wiley Blackwell. All rights reserved