Hypothalamic hypophyseal inhibitory factor (HHIF) increases intrasynaptosomal free calcium concentration

Area of Science:

• Biochemistry
• Neuroscience
• Cell Biology

Background:

• A novel sodium pump inhibitor has been isolated from bovine hypothalamic and pituitary tissues.
• This inhibitor is structurally distinct from ouabain, confirmed by mass spectrometry.

Purpose of the Study:

• To investigate the effects of the novel inhibitor on calcium (Ca2+) transport across the synaptosomal plasma membrane.
• To determine if the inhibitor influences intracellular calcium levels and homeostasis.

Main Methods:

• Measurement of ATP-dependent calcium uptake, Na(+)-Ca2+ exchange, and passive Ca2+ permeability using 45Ca2+ and fluorescence assays.
• Analysis of synaptosomal plasma membrane vesicles for Na+, K(+)-ATPase activity and changes in intracellular free calcium.
• Assessment of membrane fluidity using fluorescence anisotropy.

Main Results:

• The inhibitor demonstrated dose-dependent inhibition of Na+, K(+)-ATPase activity, correlating with increased intrasynaptosomal free calcium.
• Significant stimulation of passive Ca2+ flux (10-11 fold) and Na(+)-Ca2+ exchange (2.5-3 fold) was observed.
• No significant alteration in membrane fluidity was detected at effective inhibitor concentrations.

Conclusions:

• The novel inhibitor not only targets the sodium pump but also directly modulates Ca2+ transport systems.
• This factor may play a crucial role in regulating intracellular calcium homeostasis.
• Further research into this inhibitor's mechanism could offer insights into calcium-related cellular processes.