Growth hormone-releasing hormone triggers pacemaker activity and persistent Ca2+ oscillations in rat somatotrophs.
1. The effects of brief applications of growth hormone-releasing hormone (GHRH) to male rat somatotrophs in culture were analysed with the perforated patch clamp technique to record changes in potential or with fura-2 imaging techniques to measure variations of cytosolic Ca2+ concentration ([Ca2+]i). 2. Silent somatotrophs (n = 61) had a mean resting potential of -37 +/- 1 mV and a mean basal [Ca2+]i of 30 +/- 4 nM. Brief GHRH applications (30 nM, 40 s) triggered rhythmic action potentials (23.6 +/- 0.9 mV, 613 +/- 82 ms, 0.21 +/- 0.02 Hz) and [Ca2+]i increase (to 352 +/- 30 nM) followed by rhythmic [Ca2+]i transients (to 138 +/- 6 nM) that persisted up to 90 min after the last GHRH application. Both action potentials and [Ca2+]i transients were totally and reversibly blocked by removing external Ca2+ or Na+ or by adding inorganic Ca2+ channel blockers or nifedipine (3 microM). 3. Somatostatin (1-300 nM), carbamylcholine (0.1-1 microM) and muscarine (0.1-1 microM) each had a dose-dependent inhibitory effect, from a decrease of Ca2+ spike duration and frequency to a complete block of the GHRH-evoked action potentials. 4. The present results show that somatotrophs in culture have intrinsic membrane properties that allow them to sustain a pacemaker activity and subsequent long-lasting sequences of [Ca2+]i oscillations triggered by short pulses of GHRH and inhibited by somatostatin and muscarinic agonists.
Kwiecien
R
R
Unité de Dynamique des Systèmes Neuroendocriniens, U159 INSERM, Paris, France.
Tseeb
V
V
Kurchikov
A
A
Kordon
C
C
Hammond
C
C
D000200:Action Potentials; D000818:Animals; D001683:Biological Clocks / Q000502:physiology*; D002118:Calcium / Q000378:metabolism*; D002217:Carbachol / Q000008:administration & dosage / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D005456:Fluorescent Dyes / Q000378:metabolism; D016257:Fura-2 / Q000378:metabolism; D007987:Gonadotropin-Releasing Hormone / Q000502:physiology*; D013006:Growth Hormone / Q000378:metabolism*; D008297:Male; D018721:Muscarinic Agonists / Q000008:administration & dosage / Q000494:pharmacology; D009994:Osmolar Concentration; D010903:Pituitary Gland, Anterior / Q000378:metabolism*; D011188:Potassium / Q000494:pharmacology; D011673:Pulsatile Flow; D051381:Rats; D017208:Rats, Wistar; D013004:Somatostatin / Q000008:administration & dosage / Q000494:pharmacology; D013779:Tetrodotoxin / Q000008:administration & dosage / Q000494:pharmacology
D005456:Fluorescent Dyes; D018721:Muscarinic Agonists; D007987:Gonadotropin-Releasing Hormone; D013779:Tetrodotoxin; D013004:Somatostatin; D002217:Carbachol; D013006:Growth Hormone; D011188:Potassium; D002118:Calcium; D016257:Fura-2